Comment by ChuckMcM

Fascinating stuff, I've always been interested in tunnels and tunnel boring machines. As a teen I built an underground fortress of solitude in the unimproved desert near my house using cinder blocks for support that folks had dumped in the desert after finishing a block wall job. There was mortar mix as well so nearly two thirds of the interior walls were both brick and mortar. It was great fun to go into it during a hot day and cool off. It quickly became rather over run with snakes and scorpions who also appreciated its cool interior[1].

But my strongest memories of the place are about how good it felt to be carving a "space" out of nothing but dirt. It was almost magical in its ability to feel like creating something from nothing, even though it was just digging. Had the dot com crash not happened it was on my schedule to build a basement for my house. I got to see a two story basement (two levels down) in Palo Alto and it was really amazing to me. Almost like having an additional house to do with what you want.

[1] Always made the first visit after an extended absence pretty interesting.