At just past 7 a.m. on June 30, 1908, an explosion devastated the forest near the Tunguska river in Siberia, Russia. It was a thousand times stronger than the bomb at Hiroshima, felled nearly 60 million trees over 800 square miles, and knocked people off their feet at a trading post over 40 miles away. Local seismographs measured land movement similar to a magnitude 5.9 earthquake.
Barometers as far away as England measured the shock waves as they passed once, traveled around the globe and then passed by again. Clouds that formed over the site reflected so much light from the horizon that people in Asia and Europe could read newspapers outdoors up to midnight.
In 1921, Leonard Kulik, curator for meteorites at the St. Petersburg Museum, led an expedition to the area but didn't actually reach the blast site until 1928. He found trees charred into branchless sticks lying in a radial pattern. At the center, similar trees still stood upright: a pattern that was repeated at the 1945 atomic explosion at Hiroshima. But there was no crater and no unusual deposits. Only interviews with eyewitnesses fleshed out his observation. A 1961 Russian expedition also found nothing new. Not surprisingly, locals avoid the blast area as unlucky.
Everything from black holes to UFOs to a natural explosion has been theorized as the cause of the event. However, the most likely explanation is that a comet or meteor, 120 miles across, sped into the area at over 33,000 mph. It raised air temperatures to 44,500 degrees F before exploding over 5 miles above the ground, shooting a fireball and annihilating all traces of itself.
Had this explosion happened over Paris or London, millions would have died. But because the area was uninhabited, no casualties were reported. The Near Earth Object Office at the Jet Propulsion Labs estimates that such a body enters the Earth's atmosphere once every 300 years.