Instapundit linked to this article detailing the threat of a volcanic eruption of Mt. Rainier. Tens of thousands of Washington residents are at risk.
Sunday's Denver Post had an front page feature on a worrisome gas bubble building on the floor of Yellowstone Lake. Seems they've just started comprehensive mapping of the floor of the lake, so scientists don't know how long the bubble has been there, or how unstable it is. If it decides to let loose, the results would be devestating.
That same article mentions something called the Hebgen Lake earthquake, which occurred northwest of Yellowstone in Montana in 1959. I was unfamiliar with the event so I did a little Googling and found this, and this. The Hebgen Earthquake was a massive 7.5 magnitude, and killed 28 people, many of whom were asleep camping when landslides covered them. Fortunately the region is very sparsely populated.
A mountainside along the Madison River collapsed during the quake and slid across the river, creating a natural dam. The resulting reservoir is called Quake Lake, and is 53 meters deep.
I love this kind of stuff, and I can't believe I didn't know all this before.
A gas bubble under Lake Nyos, Camaroon, Africa, escaped in 1986, killing 1700 people and 3000 cows. Apparently they are now working on save ways to vent such lakes. I didn't remember the details, but a google of - africa lake co2 death - produced 2400 hits.