Most scientists agree that comets and asteroids pose the biggest natural threat to human existence. it was probably a large asteroid or comet colliding with Earth which wiped out the dinosaurs about 65 million years ago. An enormous object, 10 to 16 km in diameter, struck the Yucatan region of Mexico with the force of 100 megaton. That is the equivalent of one Hiroshima bomb for every person alive on Earth today.