The Underground is a big part of the planet-piece parts of the cave system, parts of it only accessible through other means. It is HUGE. There are a few caved in or flooded areas as well, one of the most infamous ones being the Brigadaken Mining Facility. The Brigadaken Mining Facility is a formerly abandoned Mining Facility, that fell into ruin after a catastrophic explosion caused a Cave In at the Main cave, completely blocking off all the other cave paths. The Brigadaken Mining Facility has a dark past as well, with many different dark things happening on the premises. Due to this, it was left to rot, as nobody wanted anything to do with it, and the Company that owned it, Karcaso Minerals, found it useless, as it would be a complete waste of money and people, to try and mine through the cave-in. It’s central cave is found 100 feet below Deeplake, 130 feet below the floor of Paradise Ravine, and 230 feet below the Edge of Paradise Ravine.
The Lost Children are a cast of kids lost within the Underground, dead, yet alive. Their only friends being each other and the lady who lives down there and stood up to take care of them.