A giant volcano might be in our distant future. From io9:

A scientist from the University of Utah has confirmed that two continent-sized “thermochemical piles” are slowly converging at the bottom of Earth’s mantle about 1,800 miles (2,900 km) beneath the Pacific Ocean. This process, says geologist Michael Thorne, could eventually lead to a cataclysmic eruption that could “cause very massive destruction on Earth.” But don’t panic quite yet. His research suggests that this super-volcano-in-the-making may not erupt for another 100 to 200 million years.

*Whew* I won’t be around then. Of course, there’s always Yellowstone. . . .

And, of course, this asteroid.