Mysterious undersea “crop circles” finally explained

Jacob T. Johansen / Wired UK

Mysterious undersea circles spotted off the coast of Denmark are not the work of aliens or spiders. Or alien spiders, unfortunately. Rather, the rings are the result of poisonous sulphides tucked into the mud coating the seafloor.

Photos of the underwater rings first surfaced in 2008. Since then, people have compared the mysterious things to crop circles or fairy rings—those enchanting little fungal designs that can sometimes spring up on your lawn. But nobody knew what the aquatic rings were, or why they were there. Some of them measured nearly 15 meters (or nearly 50 feet) across—were they World War II-era bomb craters?

In 2011, scientists determined that the rings themselves were made of eelgrass, a native type of seagrass that hosts small fish and other crustaceans. Late last year, after studying the circles, the same team found that an interaction between the eelgrasses and seafloor sulphides sculpts the mysterious shapes.