Many thanks for the fabulous comments. I feel quite blown away.

I worked underground as a shift electrician on the boring machine in the Kaimai tunnel in NZ (years ago). The rock face was 2 and a half kilometers from the tunnel entrance which was uphill with a lot of water running towards the machine. We did lose the power and flood the machine twice. And lost the air often.
There was a cave in where 4 men lost their lives. I wasn't there then but will never forget my time underground.

Richard