Yellowstone National Park sits on top of a giant volcanic caldera, or an earthen cap that covers a huge reservoir of superhot liquid rock and poison gasses. Large parts of the park were formed in previous supervolcano eruptions, the most recent of which happened about 70,000 years ago.
Now, the floor of the caldera is rising, and earthquakes in the region suggest magma movement beneath the park.
Over at Ars Technica, John Timmer explains a recent scientific paper from GSA Today assessing the risks of another supervolcano. The researchers emphasize that there are no signs of an impending eruption, so no cause for panic. But at the same time, they point out that previous eruptions came fast.
An examination of the shape and composition of crystals formed in previous eruptions indicate that the molten material experienced a rapid ascent to the surface, and didn't end up sitting in pools closer to the surface before being ejected. This suggests that a recharging of the deep magma reservoir could lead to relatively rapid eruptions, and there might not be any clear sign of the magma moving towards the surface from there that would tip us off to the coming eruptions.
All of which suggests that monitoring the deep magma reservoir is probably the best way to understand what Yellowstone might be up to.