The Tunguska event, described by an eye witness account, is another airburst theory similar to the one proposed to have brought about the extinction of the mega fauna and Tunguska also remains unresolved.
The odds are estimated that an event like the size of the explosion of an asteroid or comet that occurred over remote Tunguska, Siberia, in 1908 will occur one in 200 years to one in 1,000 years. This was the largest impact of a cosmic body with earth in human recorded history. If we're lucky, scientists will stop the next such event before its occurrence; many preventive strategies are being researched. The mystery of Tunguska is that no one has found remains of any impact object or a crater, just evidence of a 2,000 square kilometer area of flattened trees, which is why the "airburst" theory developed. A dirty snowball comet made up of dust and ice might have been the culprit. A competing theory is that a rapid combustion of methane gas was released from the swampy ground. International scientists are still investigating however.
In June 2008 Scientific American Magazine ran an article on the centennial anniversary of the event about a team that was returning to the site. The scientists were looking for evidence for their hypothesis that nearby Lake Cheko was the impact crater. An acoustic-echo sounder from an earlier trip revealed a trace of a dense, meter size object, (possibly of extraterrestrial origin?), buried in the lake sediments. A magnetic anomaly from a magnetometer survey also turned up in the same spot. What has been the outcome of the 2008 trip where the team was loaded with a suite of the latest equipment including ground penetrating radar capabilities, an underwater camera, and sediment coring devices? I'll need to scour the literature to find out if this ~100 year mystery has come closer to being solved (Gasperini, Luca et al., "The Tunguska Mystery" in Scientific American, June 2008, p. 80-86).
I include this story because it's another example of how science evolves through an interdisciplinary approach. There is more data to be gathered, and as technology advances and more research occurs, more answers will be found. Science is an enticing career choice for this reason.
(Refer to the History Channel youtube video: http://www.youtube.com/watch?v=EiXpp-i442s.)