Chasm

The chasm between the two halves of the mine has a bridge over it.

Bridge

Several Space Pirates hang out on it (and under it) and demand 10 wire to cross.

Web

50 feet below, there's a web that bridges the Chasm for level B2. For every round of movement through it a DC 10 STR check must be made to avoid getting stuck.

It's guarded by a Phase Spider

The bottom is 50' below that.