Siberia kaboom

The Tunguska Event, the biggest explosion we know about that had no human casualties, happened on the last day of June in 1908, or on the 17th because Imperial Russia was still using the old style Julian calendar. It's called Tunguska because the hypocentre (think epicentre for an earthquake but under rather than over the focus of activity) was near to Подкаменная Тунгуска, the Tunguska-under-the-Stones river, which runs through and under pebbles so you can walk across it without getting wet feet.  The grey blob in the top left corner of the picture is Lake Cheko Чеко, which some say has a magnetic lump buried in its sediment that is the only part of the extra-terrestrial object that struck the atmosphere at breakfast time and exploded.  Those same people claim that the lake was created by that lump impacting with the remote Siberian earth. The long axis of the 700 x 350m lake certainly points to the hypocentre 8km away, but that's not sufficient evidence on it's own. Other people say this theory in baloney and that the object was a dust and ice meteorite, and that accounts for the fact that there nothing could be found on the ground.  As you know, after the Chelyabinsk event a couple of years ago, hundreds of pieces from grams to half-a-ton were recovered from the holes they made in  the snow.

Whatever it was made of, something exploded several km up in the air and laid out the trees in a butterfly pattern of concentric rings (L) as if they had been scythed. 80 million trees were flattened, window-glass was shattered hundreds of km away and an atmospheric pressure anomaly was recorded in London.  This way-above-ground explosion registered Richter 5.0 on seismometers all over Eurasia. It was 200x-1000x greater in TNT equivalents than the Hiroshima bomb of 1945. Make your own conventional explosives comparison? Apart from a few fur-trappers and hunters, the actual event wasn't witnessed directly, and no dash-cams back then.  It wasn't until after WWI, the Revolution and nearly 20 years that the first qualified scientists arrived in the region to take measurements and record eye-witness accounts.  Actually, the Imperial government may well have sent people East in the immediate aftermath of the event, but any records of such an expedition went West in the turbulent times between 1917 and 1922.

Lack of data hasn't held back speculation about the cause: meteorite, asteroid, annihilating anti-matter, black hole, 'natural' H-bomb, erupting natural gas cloud, the Storm-god Ogdy.  Perhaps the most down to earth speculation makes the calculation that if the object had been 4hrs and 47mins later in arriving at the turning Earth, it would have exploded directly over St Petersburg and then we'd really have some data to analyse.