Pooled water

Any large amount of water around an underground mine is dangerous. It will use up all the available air in an un-ventilated area to the point that you will fall over and die. If you put the ventilation back in then our miner friend lives. It takes 3 days + for this to happen normally.

The other reason is people can fall and drown.

For this reason you should never enter an area with NO ventilation. You must have either primary or secondary ventilation to enter an area.