This is a pretty good site about the Tunguska Event with loads of photos and articles from different expeditions.
On June 30th, 1908, something exploded 8 km above the Stony Tunguska river. About 2150 square kilometres of Siberian taiga were devastated and 80 millions trees were overthrown. Up to now, it is not clear whether the great explosion was due to a comet or an asteroid or something else. We are searching for an answer.