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 sulfides 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 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 sulfides sculpts the mysterious shapes.

Normally, stands of eel grass grow outward in a radial pattern, forming big, circular grassy carpets. But as the grasses grow and populate shallow waters, they tend to grab onto and sequester mud that would normally continue moving along the seafloor. As the sulfide-saturated mud accumulates around the eelgrasses, the poison begins to kill the plants – starting with the older, weaker grasses in the circle’s center.

Eventually, as poisons hollow out their centers, the stands of eel grass take on a ring-like appearance, leaving the seafloor spotted with one of the shapes that’s almost guaranteed to intrigue and perplex us when it shows up in nature – whether in the ice covering the world's deepest lake or made from silk.