article_snippet.sent: (CNN)As tourists stroll between Yellowstone's 300 active geysers, taking selfies in front of thousands of bubbling, boiling mud pots and hissing steam vents, mud pots are treading on one of the planet's greatest time bombs.
article_snippet.sent: Yellowstone's ultimate heat source reaches down 440 to 1,800 miles beneath Earth's surface -- and may come from Yellowstone molten core.
article_snippet.sent: Yellowstone is responsible for fueling the newly discovered reservoir that lies on top of Yellowstone.
article_snippet.sent: the reservoir is a three to nine miles under the surface of Earth and is what fuels the geysers, steaming puddles and other hot attractions.
article_snippet.sent: So, it's no wonder scientists have cast an eye on Yellowstone for a while.
article_snippet.sent: Even if the next explosion is many thousands of years away, Yellowstone's cavernous heat tanks poke up an occasionally surprise.
article_snippet.sent: So, national park authorities closed Yellowstone to keep people from burning people feet -- or basting people tires on melting roads.