Spelunking/Caving - The World Underground After the Bomb.

[Soldier of Od]

I also thought giant mutant worms of varying sizes would be intresting.

Take a look at Mutants Down Under for some giant mutant worms (and the seismic sensors used to detect them).

I have a idea for some kind of mutant predator who hunts like a angler fish using light to attract its prey wether underground or on the surface at night.

Cool. I like it.

[Gazirra]

What about those cave ecosystems with the blind fish and bugs and stuff?

[G]

There should also be bats in large underground caverns.

They don't need light and they can fly.

[CyCo]

How far underground are you thinking? You could have some amazing stuff just under the surface that people have forgotten about. First in my mind is the subway systems around the world. A ready made system for mutant moles to live in as one example. Anyone ever seen anything about the Stalin Railway under and around Moscow? Some bloody amazing stuff build over there in the time of Stalin. And there's more than just that, there's locations around the globe, some going quite deep, especially when you get to mines. I think the deepest is over 3 miles. That's a long way down.

And there's plenty of cave systems that are mind boggling huge. There's one here in Oz, under the Nullarbor desert. People have been through there some 6 miles, and believe that's just the tip of the iceberg.