When Yellowstone’s supervolcano erupted 2.1 million years ago, it made a hole the size of Rhode Island. Makes you wonder what will happen the next time it erupts. | National Geographic TV

For this month's video update of the Yellowstone Volcano Observatory, we'd like to introduce you to a friend of ours. This friend has been monitoring changes of the ground surface in Yellowstone for decades and is so good that it can detect movement of just a few millimeters in magnitude (just a fraction of an inch). The friend? A continuous GPS station! There are dozens of continuous GPS stations throughout the Yellowstone region, and they show us that the ground in Yellowstone is always changi