Check out these pictures and descriptions of various crater lakes around the world (crater lakes are formed when a volcano basically blows it's top or implodes and fills with water)
Click!

That's a great shot of Lake Manicouagan. I remember satellite photos from our geography classes.

Some really fascinating lake's!

I just watched a show on Discovery a couple of week's ago about Lake Nyos. The villager's around there live in extreme poverty and use the lake for bathing and actually use it for drinking water! While it's still occasionally deadly to them, they've actually built up a natural tolerance to all the chemical's in the water. But the level's peak periodically causing a handful of death's.

The geologist's are predicting some major activity within the next few year's. They've tapped the layer of gases below the lake and it spray's out continuously like a fountain, but they've determined the gases are building up almost three time's faster than it's being expelled.

That's never a good thing...they could be looking at another wipeout like a bunch of years back.

An interesting feature along these same lines is the New Mexico Valles Caldera.