Centuries ago, monsters lost a war with humanity and were sealed beneath Mt. Ebott behind a magic barrier. What they built down there, in the dark, is the rest of this page.
The oldest settlement, and the first area a fallen human reaches. Its stone corridors are lined with switch-and-lever puzzles built by a king who used to live here, and its resident, Toriel, has kept it running as a quiet, deliberately gentle introduction to the world below.
A snow-covered town on the other side of the Ruins' door, home to Sans, Papyrus, and a general store that sells suspiciously reasonable winter coats for a place with no seasons. Its forest is the site of most of the game's puzzle-and-sentry gauntlets.
A humid, bioluminescent cave system strung with echo flowers that repeat back the last thing said near them. Waterfall holds the Underground's garbage dump, Undyne's house, and some of the game's quietest, saddest set pieces.
A volcanic research district built around Alphys's lab and the Core, the reactor that powers the entire Underground. Elevators, conveyor puzzles, and a noticeably rising ambient temperature mark the approach to the capital.
A shifting, half-mechanical maze built atop the Underground's reactor, wired so that its layout can be reconfigured — including, on request, live and mid-traversal. It's the last major obstacle between Hotland and New Home.
The capital city, built as a mirror of the human town monsters once lived in before the war. King Asgore's castle sits at its heart, along with the throne room and its garden of golden flowers, where most routes converge for their final confrontation.