For instance, they make the point that, according to our current, probably under-representative samples of supervolcanoes and of asteroid collisions causing similar levels of damage, supervolcanoes are ten times more frequent. Don't watch the skies; watch the ground! They show a disturbing map for all of us located near the Pacific Rim--all the supervolcanoes in the last 2 million years bar one in Italy occurred there. And they remind us that all the dramatic eruptions you might have heard of (Mount Saint Helens, Vesuvius, Krakatau, Tambora) ejected at most a few tens of cubic kilometres of magma; for the purposes of their report, the authors define supervolcanoes as those ejecting greater than 100 cubic kilometres. In other words, the biggest one you ever heard of is one quarter of the size of the smallest supervolcano. Lake Taupo took off skywards with 500 cubic kilometres of magma 26,000 years ago, but Lake Toba in Sumatra, eight to ten times larger and leaving a hole bigger than the Greater London area, is the overall winner. Thanks to what is now Yellowstone National Park on a busy day 650,000 years ago, the USGS is happy to report that "[t]he whole of the USA has been covered by ash in the geologically recent past", which I think should give anybody pause for thought.