English: On a dark September night at ESO’s La Silla Observatory, after spending the night at the telescope, astronomer Klaas Wiersema was returning to the restaurant. Most of the work at an observatory takes place at night, so it is not rare to have scientists and technicians walking around in the darkness.

This time, something unexpected happened. Suddenly, Klaas heard a loud snort behind him and the sound of massive feet chasing him. He was convinced that some kind of angry animal had emerged from its lair and was trying to hunt him down, so he ran like the wind. He couldn’t imagine what kind of beast was chasing him on the desolate slopes of the Atacama Desert, at 2400 metres above sea level, so he spent the rest of the night trying figure out the mystery.

When daylight came he went exploring, only to find that he had passed too close to a herd of wild horses visiting the observatory. The stallion was probably trying to defend its mares. He snapped this picture as proof of his close encounter the night before.

Klaas submitted this unique image to the Your ESO Pictures Flickr group. The Flickr group is regularly reviewed and the best photos are selected to be featured in our popular Picture of the Week series, or in our gallery.