Where's all this light coming from?
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.
Is there possibly lava nearby?
In my experience, light from lava ignores line of sight -- at least as of 1.8.
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.