Early in the morning of 30 June 1908 there was a massive explosion in the sky above the remote Tunguska region of Siberia. The cause of the explosion has been a subject of research and speculation ever since. The most likely explanation is that either an asteroid or a comet exploded several kilometers above the ground. No impact crater has been found.
A team of Italian scientists has been trying to explain the Tunguska event since the early 1990s (Tunguska Home Page). In 2 recent papers (pdf and pdf), they proposed that Lake Cheko, located ~8 km NW of the supposed epicenter of the explosion, may be a secondary impact crater that was created by a fragment of the exploding object. A key piece of information that would support or sink their theory is the age of Lake Cheko. However, because the Tunguska area was mostly uninhabited in and before 1908, there are apparently no reliable maps or eyewitness accounts to establish if the lake existed before then. Another vital piece of information would be provided by the identity of an acoustic reflector the Italian team detected in the sediment at the bottom of the lake and is suspecting to be a remnant of the impactor.
They also discuss their research in the June 2008 issue of the Scientific American.
On the picture below from Google Earth I marked Lake Cheko and labeled the supposed epicenter of the explosion, The coordinates for approximately the center of Lake Cheko, from Fig. 3 in Gasperini et al. (Terra Nova, Vol 19, No. 4, 245–251, 2007), are 60.964° N, 101.86° E, while those for the epicenter are from Scientific American*, June 2008, p. 83 and other sources on the Internet.
There is also this rather lackluster article about the Tunguska event in Nature News.
*The longitude for Lake Cheko given in June 2008 Scientific American is off by a degree.