The cap, which acts like a lid, was discovered 3.8 kilometers under the park’s surface Scientists discovered a magma cap under the surface of Yellowstone National Park The cap acts as a lid, making sure the pressure and heat below remain trapped Scientists say the stability of the cap likely plays a role in preventing a large volcanic eruption A magma cap that is likely playing a critical role in preventing a huge volcanic eruption has been found at Yellowstone National Park,