Lowlife

Publisher's blurb:

Lowlife is a toolkit for old-school RPGs about delving into tunnels, trenches, and mines.

Inside, you'll find:

Rules and procedures for caving, climbing, and tunneling
Hazards and dangers cavers might encounter beneath the surface
Monsters, plants, and fungi that dwell in and around the hills and caves
Items and tools to use for digging and surviving the tunnels
Generators and tools for building your own tunnel networks or modifying existing dungeons
Bits of aesthetic, flavor, and worldbuilding scattered throughout