Underground-Like Locations on the surface:
 = Old CORE -> Southern timeline border. The original CORE for the Ruins. Was never nearly as powerful as the current CORE and didn't cause any Hotland like areas. It's rusty and unstable, so watch your step.
 = Ruins -> Abandoned City, slightly maze. North of the Old CORE. Only location "underground"
 = Snowdin -> Right outside of Ruins and Old CORE. Cold temperatures + snow from duplicate CORE being deactivated and not heating the area. Settlements are there however, including Snowdin Town.
 = Waterfall -> Off to the northeast of Snowdin - Closer to the CORE, causing snow to melt. Humidity generates as well, causing the marshy like climate and geography. Contains old mining caverns.
 = Grasslands -> Off to the northwest of Snowdin - Farther away from the CORE, but it's warm enough to melt the snow. Not nearly as wet as Waterfall, it's a simple grassy area with trees scattered about and a river running through (the river leads into Waterfall as well). It's a nice camping spot.
 = Hotland -> North of Waterfall, close enough to the CORE to become dry and almost volcanic. There are hotsprings in there area, along with lava if you're low enough. The area is generally rocky, but there are some dryer shrubs scattered about.
 = Forest -> North of Grasslands, it's just a large forest. It's part of the border of the timeline at the edge.
 = CORE -> North of Hotland, stretches past New Home and is part of the timeline's border. The whole area seems like a futuristic maze, it's much more stable and enhanced than Old CORE. However, it's very hot, even hotter than Hotland. It's not advised to be there long. Unlike the canon CORE though, the bridges are closed on all sides to prevent falling. Steam may come out of the pipes every so often as well.
 = New Home -> A parallel to the Ruins, only much larger. North of the CORE, but elevated to prevent the effects of Hotland happening to New Home. There's a large castle in the center, it's surrounded by the city. Going to the edge of town.
 = Flower Medow -> North of New Home, it's a field of flowers. If you can think of a nondeadly flower, it's probably here. The end of the mewdow leads you to the timeline's border.