h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I think it was in fact some kind of nuclear device. Just because the western world couldn't create one until 1945 is no reason to think that no one else could possibly have been able to do so prior to that time!

the germans were the most advanced humans in nuclear phyisics befor world war 2 but at 1908 the human race did not know enough to cause a nuclear explosion. so what ever it was it was not man made.

It's generally thought to be a meteorite that exploded before it hit the ground. This would be consistent with the observations. The blast would have levelled more distant trees but the closer ones would have been below the blast and so would have experienced a force straight down. This stripped them of their branches and bark but left them standing.

The Tunguska explosion was probably to powerful to be nuclear - at least using today’s nuclear technology. There are many theories, that it was a miniature black hole exploding, that it was an antimatter explosion, a near light object, an asteroid, even a comet.
Being interested in unusual or lost technology I know of a type of bomb that could have caused the event - called a 'Mass Hammer' or 'Coherent Mass' bomb. This worked by converting mass to energy directly. These bombs were developed as a theoretical 'third' generation after nuclear and atomic bombs, and could produce explosions up to 100 thousand megatons in power with an explosion a hundred miles in diameter producing 'almost' no radiation. Obviously an explosion that big would be almost as big as the Tunguska explosion.

Mass hammer bombs are technically very complex though and no one has ever built one nor is very likely to soon. The thing needed to make the bomb work is also a crucial part of many scifi technologies - hyperspace engines - gravity engines - photonic drive - teleportation - even time travel and matter indention (shrinking things).

So it was either delivered from the future or more likely it was '-A-L-I-E-N-S' !!!.