I don't know how many of you know about Dwarf Fortress, but it's a excellent game. Essentially a rogue-like sims, you commandeer a group of dwarves on an expedition to create the greatest fortress that ever lived. Unlike other strategy games, DF generates an entire world, with in-depth geology and multiple elevations. Your little dwarves have to not only survive, but also survive against the harms of the outside world. After a while, you'll end up with a fortress in the earth, one filled with many levels and traps.

http://www.bay12games.com/dwarves/

It's free and in alpha so grab it if you want a taste. Keep in mind it's all ASCII for graphics (they did it on purpose), so if you're one of those people who whine about graphics and can't appreciate a real game, forget about it.

Recently one of my fortresses ended up with a artificial flood when I hit an underground water source, and messed up my floodgates. Then, water started pouring in like crazy. To save my upper levels, I had to seal off one of the lower floors, dooming two of my best miners to a watery grave. One died after a while, but another one is still kicking. Hopefully I can launch an expedition down there and divert the water away and save the other guy.