Like so many of our National Parks, Yellowstone was created by a supervolcano. The loosely defined term 'supervolcano' is used to describe volcanic fields that produce eruptions exponentially larger in scale than those of average volcanoes. To give you some context, Yellowstone Supervolcano's first eruption produced 2,500x more ash than did the Mt. St. Helens eruption in 1980 – that's what you call a double thouser with change. When these supereruptions occur, huge amounts of magma that had been collecting below the earth's surface are rapidly displaced, causing the land to collapse into the newly hollowed magma chambers. This results in large calderas, or craters, on the earth's surface. Much of Yellowstone National Park sits in the Yellowstone Caldera, a 34x45 mile crater that was created by the last of the three most recent supereruptions on earth, all of which originated from the Yellowstone Supervolcano. But the Yellowstone Supervolcano isn't quite ready to retire from the pro-eruption circuit just yet. Magma has been collecting below Yellowstone's surface since the last supereruption, filling the calderas and elevating the land by as much as three inches in a single year.
Pro Tip: Pick a long trail before you arrive and begin your hike before sunrise. Arriving early allows you to avoid the steep thirty dollar entrance fee and the congestive herds of Yellowstone's most over-bred animal: humans. By 9am the place is crawling with them. Arriving with the sun gives you access to a virtually human-less park for a few hours, and if you hit a longer trail early, you'll likely be the only person you see on it.