"Walk through the tunnel" means to enter the tunnel on one end and emerge out the other end.
"Walk across the tunnel" could mean the same thing, but only if the context establishes that.
It could also mean to enter the tunnel through a side entrance, and exit out another side entrance (i.e. the short way, not the long way); or it could mean to go across the top of the tunnel (e.g. if it's underground and the road goes over it); or variations on those themes.
In general, "through" implies entering the middle of something and then going out the other side, whereas "across" implies crossing the middle, but not necessarily going in the thing you're crossing.
ell.stackexchange.com/users/50/Martha
Across is used with surfaces, places, flat objects/areas, or things that you are "on."
Across is also used if it's important to mean "on the other side of", or "moving over" something like a river, hole, bridge, etc.
Through X is used if you are surrounded by or "in"/"inside" X.
So you go through a tunnel, but not across it.
If you say you went across a tunnel, it would mean you somehow went over and on top of the tunnel (likely perpendicular to it) and avoided going in it.
You might go through a tunnel to get across a mountain range, though.
ell.stackexchange.com/users/5144/LawrenceC
Both of these words are used to indicate a movement from one place to another.
Using " across " is somehow like using on .
We usually use across to talk about moving from one side to the other, usually on the surface.
It is used for a two-dimensional and open space (across a wall, city, sheet of paper, road and ...).
We took a boat across the river.
Looking out across the ocean, he saw land.
Using " through ", however, is somehow like using in or inside .
Imagine a place which is surrounded by something (for example a tunnel or a forest which is covered with tall trees).
When we want to move from one side of this surrounded place to the other, we use "through".
Through is used for a movement in a three-dimensional and covered place.
I am driving through the tunnel.
She loves walking through the forest.
About grass and lawn, if we are talking about a place which is covered with tall or long grass, we should use "through": When my dog runs through long grass, it’s difficult to find him.
but if we are talking about a field covered with short or small grass or lawn, we should use "across": Tomorrow at this time you'll be chasing peacocks across the lawn.
ell.stackexchange.com/users/48965/helen
If it's something enclosed , such as a tunnel or passageway, you would usually be said to walk (or pass, or drive) through it from end to end.
If it's something more or less open (but still distinctly separate from the surroundings), such as a bridge, you would usually be said to walk (or pass, or drive) across it (and the act is crossing it).
Something like an ordinary ground-level road or sidewalk you would walk (etc.) along or alongside .
If you are not moving from one end of this thing (e.g., road) to another, but only briefly occupying the same space, you would usually be crossing the road.
To cross a tunnel would usually mean that it's buried and you're above it, not within it at any time, although it would be possible to cross a tunnel from another, intersecting tunnel.
ell.stackexchange.com/users/4789/Phil Perry
To move through something, one must be able to be inside it either literally or figuratively .
Usually a container or the notion of a container is invoked.
Walk through the tunnel Drive through a rain storm Run through the forest Walk through the streets of London in each case a container is established: tunnel , rain storm , forest , the collection of London streets Across usually implies a position of being over or on top of but may mean close by , and may, but not necessarily refer to movement .
Referring to movement Walk across a bridge Fly across the country Run across the street Referring to over
The bridge goes across the river Hands Across America Referring to close by Sit across from somebody The building across from the station From your examples, one walks through a tunnel is correct walk through the tunnel across to the other side
ell.stackexchange.com/users/26439/Peter
Through is used to cross a three-dimensional place or enclosed object: like a tunnel, forest or jungle.
A forest is a three-dimensional place as it is covered with full of tall trees.
In contrast, an open field is usually empty, does not cover with trees, which is act like a two-dimensional place.
The man is walking across the field.
The man is walking through the jungle.
Credit goes to this video
ell.stackexchange.com/users/12274/Ahmad Sharif
walk across the tunnel  walk through the tunnel    Which one is correct? Can anyone explain me the exact difference between the through and across? I am really confused with these 2 prepositions.
Difference between "across" and "through"
ell.stackexchange.com
<prepositions>
ell.stackexchange.com/questions/15694
ell.stackexchange.com/users/3701/hellodear
Both of these words are used to indicate a movement from one place to another.   If something is enclosed, such as a tunnel, forest or passageway, you would usually be said to walk  through  it from end to end. "Through" implies "inside". If something is more or less  open  (separate from the surroundings), such as a bridge, you would usually be said to walk across  it. Across  is used with surfaces, places, flat objects/areas, or things that you are "on."  You walk across an open surface that you can walk on, such as a bridge. You walk through an enclosed space such as a tunnel.
You walk through an enclosed space such as a tunnel. You walk across an open surface that you can walk on, such as a bridge.