After hours of digging down, and sideways, I went up to smelt some ingots. When coming back, I went slightly down (i.e. the water level is at 92, I went to about 76) in search for iron. The ore detector found some and I started digging. Next thing I see is water rushing in. I plugged the hole within seconds, but it was too late. The entire shaft (300 units down) and all connected mines were flooded. This is not realistic. And there is currently no way of easily removing water. I'd like to see a water pump, comparable to the bucket, but with much more water capacity, to cleanup big water messes.
Also, water should raise to the top when flooding. It should not flood horizontally. A tiny hold cannot flood a 300 meters shaft within seconds.
