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Artwork of the famous Tunguska event, which occurred on June 30 1908 in the Eastern Siberian Tiaga. A stony meteoroid, some 50-60m (160-200ft) across, is believed to have entered the Earth's atmosphere before exploding at an altitude of 5-10km (3-6 miles). None of it reached the ground. This is known as ablation, caused by stresses exerted on the object as it travels through the atmosphere. The bolide is estimated to have come from the south-east, travelling at a speed of about 27 km/s (60, 000 mph). The explosion, which flattened tens of millions of trees over an areas of more than 2000 sq km (830 sq miles), was equivalent to the detonation of a 12 megaton nuclear explosion - around 800 times the power of the bomb dropped on Hiroshima in August 1945