Has anyone of you ever read the Metro series by Dmitry Glukhovsky? I am pretty fascinated by that.
When you go into e.g the subway tunnels all you are going to see is mostly spiders and some eldritch horrors. Maybe a lab but those are boring and way too common for my taste.
What I’d like to see is something other than just rock. When the portal storms hit, I assume the entire planet was affected. The biggest part of Earth is not the surface, but what is below. I could imagine there being a variety of stuff. We already have lava in the mines, which are known to be infested by otherworldly stuff. There could be lava vents and caves down below, maybe even strange ores and minerals. This would also make wilderness runs a bit “easier” since getting metals is possible this way. But this is not my main point.
I want to concentrate on the subway tunnels first, which is also why I mentioned Metro in the beginning. First of all, it is just boring tunnels currently. But we have some implicit lore:
This just screams Metro to me. The gray goo, the fungi (which appear to be different from the mycus), the twisted rails.
In real life, it is not just tunnels. There are also rooms beside the tunnels for technical equipment. There are cables, vents and ducts, too.
I could imagine people seeking shelter in the tunnels (since not all of them are lab tunnels and therefore dangerous) and therefore little settlements existing underground, maybe full of trogdolyte mutants (which already randomly roam the tunnels but are always hostile).
Since there is not much light, people would gather and farm fungi (just like in the books). There are currently more reasons to stay out of the tunnels than to enter them but for mutants with fitting traits it would have advantages. Reasons why folks would flee the surface is weather, especially portal storms and maybe acid rain and radiation, zombies, war (ever noticed all the craters, which also contain active alien bombs?) and the temperatures. It should be constantly relatively cold down there but there are no extremes. Folks could still go out to scavenge and maybe even set up little stations for solar power (since the non-vehicle powergrid is a thing finally!).
In terms of dangers, spiders seem inappropriate here. Worms or molerats look better to me. Spiders could be appropriate near vents and stuff.
The implicit lore also states these weird almost paranormal things. Tunnels being windy, especially when the roots or explosions have broken up the ceiling could definitely be a thing. There being shadow entities or something would also work.
Strong portalstorms could still affect the underground but are significantly weaker down there, but the constant exposure warped it. When exploring the tunnels, you should be mindful of more than what is in front and behind you. There being random rooms potentially containing monsters jumping at you will make it a bit more dynamic and less boring.
Worms and molerats making their den would expand the network of tunnels. Humans/mutants will also continue to dig. Collapsed, flooded or infested tunnels will also make your life harder.
The Black Gulch from Dark Souls 2 could also be a good source of inspiration for underground biomes:
It seems nightmare-ish enough for the Cataclysm.