A caldera is a geological structure formed from a volcanic crater. The word is borrowed from Spanish, where “caldera” literally translates to “cauldron or kettle.” The Spanish word came from the Latin “caldaria,” which means “boiling pot.” A caldera doesn’t have boiling liquid in it (although some are on volcanos that are still active), but many of these structures have become filled-in lakes. The Yellowstone Caldera measures 30 miles by 45 miles, and a portion of it is filled in as Yellowstone Lake.