This is a texture pack that let's you look through nearly everything in your world. It was created with the intention of making resources hidden underground easier to find, but does not serve this purpose very well because only blocks that are already exposed in some way are visible. (They must be adjacent to an empty block.) I think it makes terrain look interesting, so I figured I'd upload the texture here in case anyone else feels the same about it. You can still use it to locate caves, dungeons, and some resources, if that's the way you like to play. I'd advise against using this on an SMP server where such an exploit might constitute breaking a rule, though.
If you want to use this, you have to install it the old fashion way. (I guess the terrain selector doesn't like transparency on certain blocks.) This means opening up your minecraft.jar with a program like 7-Zip and then dropping the terrain.png into it.