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I built an underground mob spawner in 1.8 SMP that I can turn the lights on and off in. I used pistons to move glowstone over glass holes in the ceiling - whenever the lights are off the holes are covered.

But...there's light coming in from somewhere! The rest of the room is solid cobble or smooth stone, so I can't figure out why there would be so much light! How can I seal the room in darkness?

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Different angle:

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I'm guessing there is lava down those shafts to kill the monsters? That generates light. –  agf Oct 8 '11 at 3:28
What happens when you place a block somewhere near the brightest spot of one of the lit area, and then break it? It might be a bug where the lighting doesn't update. –  Kevin Y Oct 8 '11 at 3:28
@agf No, the mobs are going to go to the surface before I killses them. I'm looking around for lava outside though, like Raven Dreamer suggested. –  Michael Oct 8 '11 at 3:32
@KevinY: Looks like that may be it. I placed and broke blocks with the lights off, and it got dark. However when I cycled the lights they were lit up again. Apparently there are still lighting bugs, so maybe I hit one of them. Do you happen to know of a workaround? Otherwise I may just darken it and forget about the lighting system for now. –  Michael Oct 8 '11 at 3:42
@Michael Sorry, I don't know of any workarounds. Hopefully it'll be fixed in 1.9. –  Kevin Y Oct 8 '11 at 3:51
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3 Answers

I saw this bug in some version of 1.8, I think (but in the form of torches placed underwater leaving light behind). I didn't find any way to fix it.

It's fixed in a 1.9 prerelease.

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Is there possibly lava nearby?

In my experience, light from lava ignores line of sight -- at least as of 1.8.

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I'll have to look around a bit. There may be one that I didn't kill when exploring the caves around. –  Michael Oct 8 '11 at 3:31
@Michael Just explore directly towards the apparent source of the light! –  Raven Dreamer Oct 8 '11 at 4:05
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I have encountered this before. Kevin Y is correct. It is a lighting glitch which should be (and for me always has been) fixed by placing a few blocks in the brightest area and breaking them.

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Oh I didn't realize you had already discovered that. In regards to it coming back, how bad would it be if you just left a few blocks in the brightest area? –  John the Green Oct 8 '11 at 14:52
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