What Is the 'Hypnotic' Structure in the Sahara Desert That NASA Captured from Space?

There are few earthy land formations visible from space. One of them lies in the Sahara Dessert and is sometimes referred to as Earth's bull's eye.

The Richat Structure -- a nearly 30-mile across rock formation which Our Amazing Planet describes as "somewhat mysterious" and "hypnotic" -- was recently photographed from the International Space Station.

NASA explains that the structure, located in west-central Mauritania in West Africa was once thought to be an impact crater, but its "flat middle and lack of shock-altered rock indicates otherwise." It was also theorized to be the result of a volcano but that "seems improbable because of the lack of a dome of igneous or volcanic rock."

So what is it? According to NASA, current thoughts are that it was "caused by uplifted rock sculpted by erosion," but why it is circular still remains a mystery.

Zoom in and out on the geologic formation here.

[H/T Space.com]