Where arachnos and arachno queens spawn. You can get here by killing the bug queen in the sewers, then heading west and down.
Formerly, this was quite a nice area to farm cash in, but with the contract nerf, arachos are no longer worth the hassle. Arachnos deal slash/stab, and lots of it, so cage armor or the ability to rapidly kill the arachnos is indicated. Arachno queens deal irad damage and should be avoided.
You can scavenge tesla generator assemblies down here, which are useful for making tesla and edison gauntlets.