Ever since something generated a huge explosion over Siberia in 1908, flattening an area as big as a large city, scientists have been trying to figure out what caused it.

Among the enduring mysteries: Following the explosion, the night skies shone brightly for several nights across Europe all the way to London, 3,000 miles away.

But which type of space rock was it?

New evidence from an unlikely source -- water vapor in the exhaust plumes of space shuttles launched a century later -- points to a comet.

The potential solution comes courtesy some strange clouds that scientists have only recently begun to understand.

Noctilucent clouds are brilliant, and visible only at night. Made of ice particles, they are Earth's highest clouds, forming in the mesosphere some 55 miles over the polar regions during the summer months when, up there, it is around minus 180 degrees Fahrenheit (minus 117 degrees Celsius).