The largest city in North America, Mexico City, sits at the center of a high valley, 7,000 feet in the air, surrounded by volcanoes. Over a millennium and a half ago, a volcano called Xitle erupted. Molten lava poured into the valley and covered around 30 square miles in a bed of volcanic rock. Today,

The largest city in North America, Mexico City, sits at the center of a high valley, 7,000 feet in the air, surrounded by volcanoes. Over a millennium and a half ago, a volcano called Xitle erupted. Molten lava poured into the valley and covered around 30 square miles in a bed of volcanic rock. Today,