Wow, the first hoax I've made in a while. This one is based off of the missing "R2" level from Sonic CD that doesn't show up on the level select screen. Mercury Mine was the name this level was given in the CD Sonic 409 Beta hoax that Nemesis made a while back. In that hack, the level was incomplete, and was a black screen where you just fall down and die (like Genocide City in Sonic 2 Beta).

It's a nice mix of recolored Hidden Palace with a Death Egg background.