In the prairies near Olympia, in Washington, in northwestern United States, thousands upon thousands of grass-covered humps bulges out of the ground like an enormous bubble wrap. These humps are called mima mounds, named after the Mima Prairie, and they range in size from near imperceptible to more than two meters tall, and several meters across. Since their discovery by Charles Wilkes, a US naval officer and explorer, in 1841, these mysterious mounds have intrigued scientists and provoked curiosity, speculation, and debate.

Wilkes initially thought that the mounds were graves of ancient Indians but when he ordered his men to dig up, they found no bones. We now know that these mounds are thousands of years old but we still don’t know who or what created them. Over the years dozens of theories have been advanced implicating everything from earthquakes to glaciers to gophers to even aliens.

Photo credit: Washington DNR/Flickr