Where is that?
"Where is that?"
You mean Long Valley (AKA those mountains around the town of Mammoth) ? It's on the east side of the Sierras in California near the Nevada border south of Mono Lake and north of Bishop. It's like a smaller (but still huge 10-20 miles wide) version of Yellowstone. The Caldera is so big you have no idea you're actually sitting in the middle of a volcano.
It acts up from time to time and is still very much an active volcano just on "time out" at the moment.