Which U.S. federal agency is responsible for monitoring volcanic activity at Yellowstone National Park?

Literally, the end of the world if Yellowstone blows its top. It is a volcano with a caldera 45 miles by 30 miles, so the eruption would decimate the western part of the USA but the rest of the world would be affected by the ash shielding sunlight for years. I can’t wait for Hollywood to come up with something but there wouldn’t be a happy ending, so who wants to see that? Remember Nevil Shute’s “On The Beach”? Good book but the movie flopped. A real downer.