# Boulder
For the option, see option#boulder.

Item classes

"&nbsp;Amulets

[&nbsp;Armor

%&nbsp;Comestibles

$&nbsp;Coins

*&nbsp;Gems

!&nbsp;Potions

=&nbsp;Rings

?&nbsp;Scrolls

+&nbsp;Spellbooks

`&nbsp;Statues

(&nbsp;Tools

`&nbsp;Boulders

)&nbsp;Weapons

/&nbsp;Wands

` 

Name

boulder

Appearance

boulder

Base price

0 zm

Weight

6000

A boulder, `, is a type of huge stone object that appears in NetHack. Although boulders are considered items, they are large enough to block line of sight, and are too heavy for you to lift normally; furthermore, you cannot move to a space occupied by a boulder under most circumstances, though you can push them. Thus, in practice they are often more like a dungeon feature than an item. 
The boulder symbol is customizable through the boulder option; many players choose to change the symbol for boulders to make them more noticeable. Zero, 0, is a popular choice.

Contents

1 Generation

1.1 Monsters

2 Description

2.1 Boulders and giants
2.2 Rolling boulder traps

3 Moving boulders

3.1 Sokoban
3.2 Boulders and other traps

4 Strategy

4.1 Tactical boulder movement
4.2 Boulders and monsters
4.3 Movement in Sokoban

5 History
6 Messages
7 Variants

7.1 SLASH'EM
7.2 GruntHack
7.3 SporkHack
7.4 EvilHack
7.5 SlashTHEM

8 Encyclopedia entry
9 References

Generation
Boulders are usually randomly placed during level creation, and are generally found in corridors or as part of rolling boulder traps.
Reading a scroll of earth will cause one or several boulders to fall; if read while confused, several rocks will fall instead. Reading an uncursed scroll will cause one to fall in each square around you and on you, and reading a blessed scroll omits the boulder that would land on you - reading a cursed scroll causes a single boulder to fall on you. Monsters can also read this scroll with the same effects, though they are less likely to do so unless wearing a hard helm.
Sokoban is a dungeon branch dedicated to puzzles where boulders are used to fill pits, and is usually the reason for many players using the aforementioned boulder option - Luck penalties are usually assigned for creating or breaking boulders in this branch on an unsolved floor. See the article or the section below for more information.
Polymorphing statues will sometimes produce boulders. Boulders can be wished for one at a time, but unless you are in the form of a giant or titan, the boulder will immediately be dropped to the floor.

Monsters
Dwarves wielding pick-axes or dwarvish mattocks and other NPC monsters that dig around a level will leave behind boulders on 8.3% of diggable rock.[1]
Non-undead giants and titans that are randomly generated have a 1⁄2 chance of being given a boulder.[2]

Description
You and other monsters cannot attack through boulders, but can use missile weapons, wands, and spells to attack through them. A pick-axe, a dwarvish mattock, or a force bolt (from the spell or a wand of striking) will shatter boulders and leave behind several rocks (specifically 6 + d60)[3]. A wand of teleportation will teleport the boulder as with any other item, and a beam of polymorph will transmute one into another item. Other wands, weapons and spells, particularly the wand of digging, will have no effect on the boulder. Monsters will not target a wand of striking at boulders intentionally, and typically do so as a side effect of targeting you.
A spell of stone to flesh turns a boulder into a huge chunk of meat, which is a much less heavy object (though still quite heavy) with none of the boulder's unique properties.

The following information pertains to an upcoming version (NetHack 3.7.0). If this version is now released, please verify that it is still accurate, then update the page to incorporate this information.

Shopkeepers, aligned priests, quest leaders, and the Riders can break boulders. (commit 200cc21f)
Boulders and giants
Non-undead giants (including titans) can move over boulders, pick them up and even throw them. You can do the same if polymorphed into a giant using m, and will be given a prompt to do so if you attempt to push a boulder that cannot move in that direction. Boulders in a giant's inventory have no weight, as opposed to the 6000 aum they would weigh otherwise; you can pick one up while in giant form even if all other inventory letters are in use, and it will be assigned a hash mark inventory slot (#, also used by loadstones).
If a giant throws a boulder at you, it will land on your square regardless of if it actually hits you - you may move off the square as normal, but attempting to re-enter the square will require pushing the boulder out of the way. If you have displacement, or you are invisible and the giant throwing the boulder cannot see invisible, a thrown boulder that misses will instead land at the square where the giant thought you were.

Rolling boulder traps
Main article: Rolling boulder trap
The rolling boulder trap consists of a trigger square and a single boulder placed out in the open, usually in the same room as the trap, and requires a clear path to trigger.[4] If you or another monster steps on that square, the boulder will roll to the opposite side, usually hitting you or that monster along with anyone else in the boulder's path;[5][6] being hit by a boulder deals d20 damage.The boulder will stop rolling if it runs into something that obstructs it - a boulder that crashes into another boulder will send the second boulder rolling some distance.
The boulder will always roll to the other launch square on the opposite side if possible - as long as the boulder is on a launch square when the trap activates, the trap can set it rolling and will do so on repeated triggers.[7] A boulder trap will not trigger its boulder to roll if the path is obstructed by a wall, or the boulder is moved by anything other than the trap itself. A boulder set rolling by this trap can disappear midway through rolling if, for example, it fills a pit, hole or trap door, or it is caught by a giant that was in the way. Stepping on a rolling boulder trap that cannot trigger a boulder will cause the trap to disappear with no other effect.

Moving boulders
You (and other non-giant monsters) cannot travel past boulders without phasing - if you try to move onto a square containing a boulder, you will push it to the next square in line if possible; if there is more than one boulder on that square, you will try to push each boulder off in turn until one of them does not move.[8][9] Pushing boulders this way exercises strength. If you are riding and try to move onto a square with a boulder this way, your steed will push the boulder.[10] You cannot push any boulders if you are levitating, riding a levitating steed on else are on the Plane of Air, due to lack of leverage.[11] You also cannot push boulders if you are in a tiny-size form, though you can still squeeze past them.[12]
In the diagrams below, the # represents where the boulder will move:

@&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;@&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;@&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;#&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;#&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;#&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;`&nbsp;&nbsp;&nbsp;&nbsp;`&nbsp;&nbsp;&nbsp;&nbsp;`&nbsp;&nbsp;&nbsp;#`@&nbsp;&nbsp;&nbsp;`&nbsp;&nbsp;&nbsp;&nbsp;`&nbsp;&nbsp;&nbsp;&nbsp;`&nbsp;&nbsp;&nbsp;@`#&nbsp;&nbsp;#&nbsp;&nbsp;&nbsp;#&nbsp;&nbsp;&nbsp;#&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;@&nbsp;&nbsp;&nbsp;@&nbsp;&nbsp;&nbsp;@&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
Boulders that have a wall, door or other obstruction (such as a monster or another boulder) in the direction you are pushing will not move, which does not use up a turn; you cannot push a boulder through a closed door, and also cannot push a boulder diagonally through any door. You can "squeeze" through onto the same square as an obstructed boulder if you are not on a steed and either have an empty inventory (not counting any gold) or are at least 850 units below your carrying capacity.[13] If a boulder becomes stuck in a corner, then you can not move it at all even if you squeeze onto its square, unless you are able to dig or otherwise maneuver around it.
If a boulder is blocking a corridor and is pushed to a square that branches off, you can maneuver diagonally around it:

&nbsp;&nbsp;#&nbsp;&nbsp;##`##
More scenarios and applicable tactics are discussed in the section on boulder movement below.
Pushing a boulder into a hole, trap door or either type of pit will usually plug it and turn that square into normal floor[14][15] - objects in a plugged pit are buried, while objects on or near the square of a hole or trap door may be knocked through before it is plugged.[16]
A boulder that is pushed, rolled or thrown into a moat will fill it 9⁄10 of the time and create a floor tile that you can move across, and the remaining 1⁄10 of the time it will disappear with a splash that wakes up monsters.[17][18] This probability is reversed for lava, which causes the boulder to fill the square only 1⁄10 of the time, and disappear otherwise[17] - lava will splash onto you in either case, which deals 3d6 fire damage to you, burns away any slime and wakes up monsters;[19] fire resistance reduces this to d6 damage, and the damage can also be reduced by the half physical damage property.[20]
Boulders have a 3% chance of falling through a down stair if another thrown object lands on their square; this includes gold, but not rocks and gems.[21] You can also push them onto a teleport trap or level teleporter, though there is a very low chance of it landing on the down stair's square.

Sokoban
Main article: Sokoban
Sokoban is a branch with special rules for boulder pushing. Namely, you cannot push boulders in Sokoban diagonally until a floor is cleared. [22] Additionally, there are Luck penalties for the following boulder-related actions, if the floor is not yet solved:[23][24]

Squeezing past boulders
Moving onto or picking up boulders
Breaking, polymorphing, or casting stone to flesh on boulders
Creating boulders with a scroll of earth
Boulders and other traps

A user has suggested improving this page or section as follows:
"Account for trap doors, land mines and some other interactions."

While boulders can be used to fill holes and pits (including spiked pits), the exact interactions and effects depend on a few factors, including: whether the boulder or the pit/hole occupied the square in question first; the source of a boulder that is moved into a pit or hole; the source of a pit or hole that occurred in a square with a pre-existing boulder; and where you are standing in relation to the pit or hole.
Pushing a boulder into a pit or hole removes the boulder and the plugged trap, regardless of whether or not its location was known to the player character; items on the square of the pit are buried, while objects on or next to a hole's square may be buried or knocked through.[16] Boulders that land on the square of a pit or hole as a result of being thrown, dropped, sent from the sky by a scroll of earth, or else rolled via boulder trap will plug that trap and remove it, burying items as normal.
If you are trapped in a pit and a boulder is dropped or rolled in on top of you, the effect varies depending on if you are a giant or not. If you are a giant, the pit is filled and you are instantly freed from it without any further effect - if you are not a giant, the pit will not be filled, and there is a chance during each attempt to escape the pit that it is filled by the boulder, leaving your leg is stuck in a leftover crevice that you must pull free from. Zapping a wand of teleportation down or applying it while the boulder is still on that square will teleport it elsewhere, allowing you to try escaping the pit as normal. Holes that are filled while you are on their square will be plugged, just as if you were standing next to that hole.
Applying a charged drum of earthquake can create a pit underneath a boulder, which immediately fills the pit and also buries every item on that square underneath it[25] - interestingly, if there are multiple boulders on that square (e.g., usually from a giant dropping them all at once), they will also be buried, minus the boulder that fills the pit. If you are on top of a boulder and zap a wand of digging down on that square, you will create a hole or pit depending on the whether the level is diggable, and the trap is plugged immediately before anything can fall through or into it.[26] If a boulder is already present on a square with a pit, and you manuever into that square and zap a wand of digging down, the boulder will fill the pit instead of creating a hole, with two different messages possible.[26]
Though lacking any practical use, it is possible to get a boulder on top of a pit or hole outside of wizard mode: Drop a statue on top of the square via levitation, or by avoiding falling into the pit or hole when moving onto its square (which occurs with a 1⁄5 chance).[27] If you then polymorph the statue into a boulder, it will drop into and fill a pit, but not a hole - throwing other objects at a boulder that is placed on the same square as a hole in this way will cause it to fall through the hole and land on a lower level, as with boulders on a downward staircase.
You cannot dig down with a pick-axe while on a square with a boulder, and breaking a wand of digging will not create a hole or pit under a boulder - both are due to special checks in the code.[28][29]

The following information pertains to an upcoming version (3.7.0). If this version is now released, please verify that it is still accurate, then update the page to incorporate this information.

Account for cases affected by commit a18859d - monsters that zap a wand of digging downward on an undiggable floor create a pit just as you do, and will usually fall into it immediately.
Strategy

A user has suggested improving this page or section as follows:
"Expand this/other sections if possible."

Main article: Boulder fort
 Four of the boulders in this screenshot were blocking the long corridor, but through deft squeezing and pushing the hero pushed them into rooms. If that boulder in the lower-right had started at that position, it would likely be part of a boulder trap.
Boulders are often used to protect stashes or polytraps by pushing one on top of the desired square - boulders can also be arranged near a square to use as a "fort" that can keep most monsters away, minus hostile giants and phasing monsters like earth elementals. Boulder forts are usually created by reading a non-cursed scroll of earth, with blessed scrolls preferred to avoid taking damage in the process. 

Tactical boulder movement
Boulder placement is often an important consideration, whether simply traveling through the dungeon or else fighting and/or evading hostile monsters.
These two boulders below are stuck against a wall, and you may only push them in two directions:

-----------&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;------&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;|...`.....|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;|....|&nbsp;&nbsp;&nbsp;&nbsp;-------|.........|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;#####..`...####......||.........|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;#&nbsp;&nbsp;&nbsp;&nbsp;------&nbsp;&nbsp;&nbsp;&nbsp;|.....|---.-------&nbsp;&nbsp;&nbsp;###&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;-------
These boulders are completely stuck, and require digging (or squeezing in the second case) to move:

----&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;----|`..&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;.``.|...&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;....
However, it may still be possible to move them if there is a door that can be opened on the other side. For example, the door next to the boulder in the first diagram below is hidden, and once found and opened the boulder can be pushed through it. In the second diagram, only the first boulder can be pushed from other side of the door opened, which enables movement of the second.

----&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;-+--S`..&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;.``.|...&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;....
Boulders and monsters

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;#######@`I###&nbsp;&nbsp;&nbsp;#&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
In the above diagram, the space opposite the boulder in the corridor is occupied by an unknown monster. In most situations, you can wait a turn or two using . or s and then try pushing again to see if the monster has moved - if so, you can gradually push the boulder down the corridor until you reach the fork and move around it.
This is not the case in the next diagram below, where you must either squeeze past the boulder or remove the monster in question:

###@`I#######
In cases such as the two above, you should also determine whether or not the monster on the other end is hostile if possible - telepathy, monster detection and warning are generally useful methods to this end. If you discover a peaceful monster on the other side that you do not want to kill, you may want to try another route or else squeeze past the boulder, then push it back the other way. A magic whistle can bring pets that block a boulder's path to your side. If all else fails, or else it is sufficiently early in the game, you may opt to drop most of your inventory with D and then squeeze onto the boulder to see who or what is obstructing you - squeezing is especially useful if there is a door, hidden space, or another boulder on the other side.
Once you confirm a hostile is blocking a boulder's path, or you are willing to take the alignment penalty for killing a peaceful monster, you can squeeze past the boulder to fight them with whatever weapons you can bring onto the square (if any), or stay where you are and employ any ranged weapon or attack at your disposal: thrown projectiles, most spells and wands, etc. Throwing the wielded Mjollnir or a tethered aklys is somewhat risky as there is a 1% chance of the weapon failing to return, though a spare aklys can save you the trouble of having to retrieve other used projectiles from behind the boulder afterward.
When faced with two consecutive boulders in a corridor, as shown below, you can squeeze past one to push the other:

##@``######@`######`@`#####`#@`#
Whenever squeezing through boulders, be sure to retrieve your dropped inventory as promptly as possible; one quick method is to toggle autopickup via @ (usually Shift&nbsp;+ 3) if you have not set any exceptions, then move onto the square where you dropped your items.

Movement in Sokoban
The boulder movement rules in Sokoban are a source of complication in solving the puzzle, as monsters can appear within the pit- or hole-filled sections and obstruct your path while pushing boulders:

------------..@`I^^^^^..------------
Here, squeezing yourself onto a boulder's square will induce a -1 Luck penalty. The monster in question will try to avoid falling through the hole next to them if possible, and so is likely to remain stuck unless they are killed or decide to finally move and fall through. This is especially troublesome if the monster is peaceful, since you most likely will have to take an alignment penalty alongside the Luck penalty - pray that a co-aligned unicorn does not appear between your boulder and the holes! When possible, make sure that your path leading to the holes is clear before resuming pushing boulders - magic whistles are ideal to keep pets by you, and strategically placing known cursed items can ward them off from wandering near the holes.
In addition to cornered boulders, a 2x2 square composed of boulders and/or wall squares is likewise considered stuck by branch rules, as moving them diagonally is not permitted unless the floor is already solved:

......&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;...|....``..&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;---`....``..&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;..``........&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;......
Be very careful in Sokoban to not stick boulders in the above manners, unless you are able to solve the level with the remaining boulders.

History
Boulders have been present in the game since Hack 1.0. From this version to NetHack 2.3e, they were known as enormous rocks.

Messages
With 

<great little=""> effort, you move the boulder.
You moved into a boulder and pushed it; the word "little" occurs if you are in the form of a giant or titan.
You try to move the boulder, but in vain.
The boulder you are pushing is obstructed.
Variants
SLASH'EM
In SLASH'EM, rays from the spell of dig or wand of digging will vaporize boulders.
Tiny monsters can squeeze past boulders, while hiding monsters such as cave spiders and slithy monsters such as snakes can use boulders to hide.

GruntHack
In GruntHack, player giants can move over and pick up boulders as other giants can.

SporkHack
SporkHack adds ceiling collapse traps that may drop several boulders and rocks on and around the trap; they are capable of dealing immense damage that can potentially kill high-HP characters even if they are wearing a hard helmet.

EvilHack
In EvilHack, boulders are stackable. Giants are also a playable race in EvilHack, with the same ability to pick move over and pick up boulders - a boulder carried by a player giant weighs 6 aum rather than 0, in order to prevent them amassing an endless amount; boulders can also be stacked like other projectiles.

SlashTHEM
In addition to SLASH'EM changes above, SlashTHEM also includes ceiling collapse traps from SporkHack; giants are also a playable race, with the ability to move over and pick up boulders as in other variants.

Encyclopedia entry

I worked the lever well under, and stretched my back; the end of the stone rose up, and I kicked the fulcrum under.  Then, when I was going to bear down, I remembered there was something to get out from below; when I let go of the lever, the stone would fall again.  I sat down to think, on the root of the oak tree; and, seeing it stand about the ground, I saw my way.  It was lucky I had brought a longer lever.  It would just reach to wedge under the oak root.
Bearing it down so far would have been easy for a heavy man, but was a hard fight for me.  But this time I meant to do it if it killed me, because I knew it could be done.  Twice I got it nearly there, and twice the weight bore it up again; but when I flung myself on it the third time, I heard in my ears the sea-sound of Poseidon.  Then I knew this time I would do it; and so I did.

[ The King Must Die, by Mary Renault ] 

References

↑ src/dig.c in NetHack 3.6.6, line 1328

↑ src/makemon.c in NetHack 3.6.6, line 183: Special case for titans and most other giants

↑ src/zap.c in NetHack 3.6.6, line 4720

↑ src/trap.c in NetHack 3.6.6, line 398

↑ src/trap.c in NetHack 3.6.6, line 1534: Handling for players activating the trap

↑ src/trap.c in NetHack 3.6.6, line 2690: Handling for monsters activating the trap

↑ src/trap.c in NetHack 3.6.6, line 1771

↑ src/hack.c in NetHack 3.6.6, line 105

↑ src/hack.c in NetHack 3.6.6, line 302

↑ src/hack.c in NetHack 3.6.6, line 310

↑ src/hack.c in NetHack 3.6.6, line 124

↑ src/hack.c in NetHack 3.6.6, line 371

↑ src/hack.c in NetHack 3.6.6, line 367

↑ src/hack.c in NetHack 3.6.6, line 210

↑ src/hack.c in NetHack 3.6.6, line 224

↑ 16.0 16.1 src/dokick.c in NetHack 3.6.6, line 1537

↑ 17.0 17.1 src/do.c in NetHack 3.6.6, line 60

↑ src/do.c in NetHack 3.6.6, line 90

↑ src/do.c in NetHack 3.6.6, line 106

↑ src/do.c in NetHack 3.6.6, line 111

↑ src/dokick.c in NetHack 3.6.6, line 1431

↑ src/hack.c in NetHack 3.6.6, line 145

↑ src/hack.c in NetHack 3.6.6, line 361

↑ src/hack.c in NetHack 3.6.6, line 374

↑ src/music.c in NetHack 3.6.6, line 332

↑ 26.0 26.1 src/dig.c in NetHack 3.6.6, line 816

↑ src/trap.c in NetHack 3.6.6, line 942

↑ src/apply.c in NetHack 3.6.6, line 3417: Breaking the wand calls dig_check in dig.c

↑ src/dig.c in NetHack 3.6.6, line 226: dig_check checks for boulders here; do_earthquake is music.c does not call this function

</great>