Dwarf Fortress -

In Dwarf Fortress , digging "deeply" is a core mechanic that transitions the game from a surface-level survival simulation into a complex, high-stakes exploration of the subterranean world. As you descend, you move through multiple geological layers, discovering increasingly rare minerals, unique ecosystems, and dangerous "Fun"—a community euphemism for the catastrophic events that inevitably lead to a fortress's downfall. The Vertical Journey

The depth of your world is determined during generation, often reaching down to levels below the surface. Dwarf Fortress