When Alan Hicks and his family decided to take a stroll through Dartmoor National Park in April of 1977, they never could have anticipated the mysterious and tragic scene they would find.

As they made their way through Hollowcombe Bottom, a part of the park just outside Postbridge, they approached a herd of about 15 Dartmoor ponies that appeared to be sleeping. Except upon closer examination, the family soon realized they weren’t intruding on a massive pony slumber party. Every equine was dead.

Dartmoor Ponies are commonly found in the park—after all, they’re native to the area and even inspired the park’s logo. It’s even possible to stumble upon an occasional dead pony. However, it’s unusual to find an entire deceased herd.

No one knows for sure what happened to the animals. They were found with broken limbs and bloodied flesh. It’s said their bodies decomposed at an unnaturally fast rate. Theories range from parasites to lightning strikes to flash floods to satanic rituals.

Some people even blame aliens, saying the most plausible explanation is that a low-flying UFO swooped in and created a vortex that doomed the ponies. Extraterrestrial enthusiasts are convinced otherworldly beings are behind the curious deaths, though their motives remain unclear.