The Tunguska trees are devastated by an explosion that occurred close to the ground, as evidenced by the butterfly pattern of trees knocked sideways. It was a huge cloud of well mixed methane and air, equivalent to all the natural gas being piped about in the US at any given time, and the burn spread around this cloud or that, under and over and around, until a particular pocket of well mixed methane and air had no where to go with its heat since the burn was all around. Itself burning, the heat ramped the combustion up to the explosion level. Methane gas occurs naturally, a result of the decomposition of organic materials. Landfills must vent this or experience explosions.
Some humans know they can light and briefly burn their farts.
Humus or accidentally buried organic material is a source of methane gas, and if not vented, this attempts to rise, being light, and will pool if trapped. Siberia was once lush, a fact the carcasses of mastodons reveal, as their bellies are full of grass. Flash frozen and covered with volcanic dust, organic material lies as a potential . Where Siberia may appear to be a frozen wasteland, the center of the Earth is hot, and decomposition of trapped matter, proceeding slowly but over a long time, can accumulate a large, trapped pool of methane gas. Released Due to a Shift in the Earth's crust and encountering a raise in temperature sufficient to act as a spark, this would explode, with the size of the explosion in proportion to the volume of violently venting gas.