Mysterious sea circles that appear on the seabed near Denmark are created by eelgrass, and are a result of agricultural runoff. Hope you’re not too disappointed that it’s not fairies or aliens.

Eelgrass grows in clumps, with older plants in the middle and younger ones near the edge. The build-up of toxic sulphides from a combination of bacteria and agricultural runoff kills off the older and younger plants, leaving perfect circles of mid-aged grass.