© NASA/ISS programThe Aorounga structure is a roughly 8-mile-wide impact crater left behind by a "city-killer" asteroid that slammed into Earth.
The Aorounga structure is a 7.8-mile-wide (12.6 kilometers) impact crater located in the southeast Sahara in northern Chad. The crater is made up of two rings that give the structure its eye-like appearance: An inner ring with a central hill, or uplift structure, that looks like a pupil; and an outer ring that looks like an eyelid. The rings rise around 330 feet (100 m) above the surrounding ground but have been heavily eroded over time — similar to other ancient impact craters — and were likely even taller and wider originally.
Experts believe that the structure formed around 345 million years ago and was likely created by a meteor around 2,000 feet (600 m) across, according to estimates from the Lunar and Planetary Institute. An impactor of this size, known as a "city-killer" asteroid, would have caused widespread damage across the north of Africa and may have even triggered climatic effects on a global scale.
The structure also has several dark lines running across both of its rings, which are sections of massive ridges, known as yardangs, according to the U.S. Geological Survey. These ridges, which can reach up to 100 feet (30 m) above ground level, stretch across the surrounding areas for dozens of miles, as you can see in the 2016 astronaut images below.
