So, a hundred years ago today,* on the morning of June 30th 1908 (June 17th in the Julian calendar, which was still in use in Russia at the time), something happened in the sky over Siberia, though exactly what is still a point of contention even now.
Since the area was so remote, twenty years passed before the first expedition arrived at the site to try to figure out just what had happened. Piecing together the physical evidence (trees knocked down in a thirty-mile diameter area but still standing in the epicenter), eyewitness accounts (from traders and herders close enough to see the fireball but far enough away to survive the blast), and indirect evidence reported hundreds and thousands of miles away (a train almost derailed by seismic waves, air pressure graphs, odd nighttime luminescence), led to the conclusion that something had exploded a few miles up in the air. The generally accepted theory was that it was a meteorite, but lack of a visible crater and some other anomalies have led some to speculate that it was a comet; more exotic theories ascribe the Event to a UFO, or the Earth's collision with a black hole, or a speck of antimatter. The theories are interesting in themselves for what they reveal about our evolving obsessions over the last century; here are a dozen of them listed in chronological order, taken from Surendra Verma's THE MYSTERY OF THE TUNGUSKA FIREBALL :
#1: a meteor 
#2: a comet 
#3: anti-matter 
#4: a UFO 
#5: a laser beam from space 
#6: a mini black hole 
#7: a massive lightning ball 
#8: a plasmoid ejected by the sun 
#9: a geometeor erupting from the earth itself 
#10: a Mad Scientist  (specifically, Nikola Tesla's death ray)
#11: a massive methane venting  (the epicenter is a swamp/bog, after all)
#12: mirror matter  (like antimatter, but different)
In retrospect, it's surprising that it took so long for the 'Mad Scientist' theory to be put forward, or that the UFO idea came so early (two years before the 'flying saucer' craze started). Despite so many theories, two definitely dominate the discussion: the meteor (based on eyewitness accounts and sheer probability) and the comet (to explain the lack of a crater or visible debris). And since the fuzzy edges of science are where the new mythologies flourish, we'll probably get more ingenious theories before the meteor/comet debate gets definitively settled.
In the meantime, time I think to dig out my favorite story based on the Tunguska Incident, Dunsany's "A Big Diamond", from THE TRAVEL TALES OF MR. JOSEPH JORKENS  (far better than D. R. Bensen's AND HAVING WRIT ) while listening to Alan Parsons (without The Project)'s 'Return to Tunguska' (from A VALID PATH ).
*Nt: drafted on Monday, June 30th, though not posted until Wend. July 2nd