To understand what went on at Sago, it's worth a little review on how underground mining operates.  Here's a short course in underground coal mines, along with speculation on what happened at Sago, and some direct finger pointing at the mine's owners.

Anatomy of an Underground Mine

Back in the day, underground mines meant a lot of drilling, blasting and shoveling.  Not any more.  These days, there are basically two types of underground mines: continuous miner operations, and longwall units.   The "miner" in continuous miner is actually a machine.  It's a heavy metal cart with a large spool-shaped arrangement at the front that's studded with spikes set with industrial diamond.  The spool turns, the machine advances, and the ground up coal comes out the back where it generally goes straight onto a conveyer belt and right out of the mine.  A longwall machine looks somewhat similar, but the longwall has a set of metal roof supports attached.  The big difference is in the way they mine.