More than half a billion years ago, a meteorite struck Earth near the Antarctic Circle, leaving a divot several kilometers in diameter. In the hundreds of millions of years that followed, the movement of Earth’s tectonic plates brought that cratered piece of crust far into the Northern Hemisphere.
Today, the Söderfjärden impact crater occupies a 22 square-kilometer piece of coastal real estate in western Finland near the Gulf of Bothnia, the Baltic Sea’s northern arm. The OLI-2 (Operational Land Imager-2) on Landsat 9 acquired this image of the hexagonal-shaped crater in September 2024. The feature stretches more than 5.5 kilometers (3.4 miles) from east to west and is divided into many agricultural fields.
In its Scandinavian locale, however, Söderfjärden has not always been on dry land. During the Last Glacial Maximum, around 20,000 years ago, a thick, heavy sheet of ice covered the area and pressed the land down hundreds of meters. Now free of that weight, the land has been bouncing back with some of the highest rates of uplift, or glacial isostatic adjustment, on Earth. New ground emerges from the sea every year.
It’s only in recent centuries that the crater began to appear from beneath the water. It first manifested as a bay (Söderfjärden translates to “south bay”) where people reportedly fished for pike and perch until the 18th century. As the land continued to rise, the crater got progressively drier and eventually transitioned into a wetland area and then an inland depression.
At first, sedges and reeds thrived in the boggy ground, and people would harvest the vegetation for livestock feed. In the early 19th century, pumps were installed to drain Söderfjärden and increase its cultivatable area. Hay barns then proliferated across the crater, peaking at 3,000 in the 1940s and 1950s, according to the Söderfjärden visitor center.
Today, most of the land is used to grow cereal crops such as barley, wheat, and oats. The fields are also valuable to various bird species and famously attract thousands of common cranes during their spring and autumn migrations. Still low-lying ground, the crater continues to be pumped of water.
Söderfjärden interests planetary scientists because of its geometric shape. Some have described the Finnish crater as “the best sample of a hexagonal impact structure on Earth.” Polygonal impact craters, defined as having straight sections along their rims, are a relatively small subset of all impact structures, which tend to be circular. Nonetheless, polygonal impact craters mark the surface of planets, asteroids, and moons throughout our solar system—from Mercury to Pluto’s moon Charon. Instruments on NASA’s Voyager 2, Cassini, MESSENGER, and other spacecraft have imaged many such craters on objects in our solar system.
Scientists believe polygonal impact craters are shaped by underlying geology and that their straight segments form where structures such as faults or other fractures already exist. As a result, the shapes can provide evidence of the geologic past of planets and moons that might otherwise be hidden from view.
NASA Earth Observatory image by Wanmei Liang, using Landsat data from the U.S. Geological Survey. Photo by Timo Kyttä. Story by Lindsey Doermann.
