Some say the cause was one of Tesla’s experiments.
Some say an exotic craft exploded or had some effect just above the surface there.
David Hatcher Childress claimed in one of his ridiculous but entertaining books that Tesla’s test of his Wardenclyffe Tower wireless transmission of electricity system corresponded with the Tunguska blast. There’s literally no way that could have happened, since the facility was in foreclosure in 1908, and Tesla had no money. In addition, knocking down all the trees for miles — in the Middle of Nowhere, Siberia — would have required far more energy than could be broadcasted from the tower. Also, there’s the matter of why a firing-up of what was ultimately a radio transmitter in upstate NY would cause an explosion in Siberia — there is no connection at all.
The book “The Fire Came By” regarding the exploding UFO angle was an interesting read (that was during my college days) but the case isn’t compelling. The exploding UFO idea originated with a Russian circa 1960, but he’s not the author of the book. It’s probably around here somewhere, in the chaos and disorder of my home. ;’)
One detail from “TFCB” that’s of interest here is that the incoming bolide that impacted near Tunguska was seen burning its way across the skies of India.