Two years after the 1883 eruption of Krakatoa, the Indonesian supervolcano, Robert Leslie published a note in the journal Nature describing wispy blue filaments in the night sky. He is now credited with the discovery of noctilucent clouds (NLCs).
Photograph from the International Space Station, positioned 340 km over western Mongolia on July 22, 2008. Clouds estimated to be 83km above earth (at the edge of space). Credit to NASA at http://science.nasa.gov/headlines/y2008/25aug_nlc.htm
According to Gary Thomas, atmospheric scientists at the University of Colorado, the clouds are thought to be spreading and their first sightings coincide with the Industrial Revolution.
Thanks to Willem for the link.