Jonah gives a hint about that.
As soon as the upper tunnels (Lower Underrail) are cleared, which you can play an important part in by getting that drill rotor circuit board, traders get clearance to sell you TNT charges with which you can clear away the rocks and progress through the caves.
It's really just a nice place to find some rare poisons and a bunch of fluff, with some additional connections between places you're not meant to see before anyways.
