Isn't that just what makes it the best?
I remember a while back in early 2010 version I wanted to check out the caverns. I'd heard how dangerous they could be so I built a foodgate system where I had two sets of floodgates that created a safe room between the tunnels and my fort which could be flooded with water on command. I prepared my rag tag military/explorers and opened the gates and started down, immediately set upon by three blind cave ogres as well as some other random stuff from down there (crundles, elk birds or something, and cave rats). I slammed the gates shut but, of course, the ogres busted them all down and entered my base, it was chaos but ultimately we were able to kill them taking great losses, most notably my accountant (whom I always name after my brother) had both his arms torn off (one at the elbow, one at the shoulder) and amazingly enough, over time, survived! He was useless as a dwarf but he did get better and spent most of his time wandering around my great hall..
So what makes this such a grand experience is that I decided to explore the caverns again (I had walled them off after driving off the ogres), I open them up and again, ogres, I attempt to take them down again until I notice that there is water in my base, one had gotten up top and destroyed the floodgate blocking off a river (which diverted it down to my water trap) and my fort was flooding from the top down, there was no way out, my miners were dead and I had no idea where their equipment was, it was the end of days..
As more and more creatures poured out of the depths my armless accountant found off a giant cave rat by biting him into submission (the rat ran off, wasn't killed). I was able to seal off the flooded sections as well as the areas with creatures in them but it was all over, I had no more ability to dig, there were monsters in my base and there was no way out through existing channels, it was quite a ride however.
That is one of the many, but perhaps the primary, reasons I love this game.
-MB