On June 30, 1908, in the basin of the Podkamennaya Tunguska River, there occurred a powerful explosion most likely to have been caused by the air burst of a large meteoroid or comet fragment at an altitude of 5–10 kilometers (3–6 miles) above the Earth's surface. The force of the explosion is estimated to be about 10–15 megatons. It flattened more than 2,000 square kilometers (500,000 acres) of pine forest and killed thousands of reindeer.