Several years ago, designer Tyler Nordgren created a series of retro National Parks-style posters celebrating the wonders of our Solar System, from the volcanoes of Jupiter’s moon Io to the geysers on Saturn’s moon Enceladus. But there was one striking feature that didn't make it on the list—because it hadn't yet been discovered. Introducing: Mercury’s Great Valley.

The vast scar on the surface of the planet closest to the sun is 250 miles wide, 600 miles long and 2 miles deep, making it the largest valley on the planet, according to a press release from the American Geophysical Union. The canyon even dwarfs Earth’s largest valley, the 277-mile long Grand Canyon (however, 1,860-mile-long Valles Marineris on Mars is still the Solar System’s grandest canyon in terms of length).

The Great Valley was discovered using images captured by NASA’s MESSENGER spacecraft, which orbited Mercury between March of 2011 and April of 2015 before (intentionally) crashing into the surface of the planet. Tom Watters, a senior scientist at the Smithsonian National Air and Space Museum, and his team found the valley while analyzing data from that mission. Their research is detailed in the journal Geophysical Research Letters.

While the size of the Valley is certainly impressive, what it reveals about the composition and history of Mercury is even more significant. Unlike the Grand Canyon, which was created by the flow of water, or East Africa’s Great Rift, which is caused by two tectonic plates pulling apart, the Great Valley appears to have been created from another process, Watters says.

On Earth, tectonic plates constantly pull apart and smash into one another. But Mercury has one single plate, called a lithosphere, that acts as a shell around the planet. As the planet's core cools, the surface contracts and buckles. One of the result are “fault scarps” like Enterprise Rupes and Belgica Rupes, the two giant cliffs bordering the Great Valley.

In a previous study released in late September, Watters and his team showed that Mercury has many small, recently-created fault scarps, indicating that the planet’s lithosphere is still actively moving and changing. In other words, like Earth, it's still tectonically active. Readings from MESSENGER also showed that the planet has had a magnetic field for 3.6 billion years, an indication that at least the outer portion of its molten core is still warm.

The fact that Mercury is tectonically active was a surprise, says Watters. Previously, researchers believed Mercury’s core froze long ago and the planet was a static chunk of rock. “The conventional wisdom has been the smaller the size of a body, the faster it cools off,” says Watters. “The notion was that Mercury must have cooled off long ago given its size. But in order to keep its magnetic field going for billions of years, it must have been cooling much more slowly and must still have currently active tectonics.”

“This is why we explore,” NASA Planetary Science Director Jim Green says in a press release. “For years, scientists believed that Mercury’s tectonic activity was in the distant past. It’s exciting to consider that this small planet—not much larger than Earth’s moon—is active even today.”

Watters points out that Mercury’s one-piece lithosphere shows that the four rocky, inner planets in our Solar System all have very different histories. Earth and Mercury are both tectonically active but in very different ways. Venus may or may not have tectonic activity, but if it does the process is different also since there is evidence that its crust is very young, only 1 billion years old or less. Mars may have had some element of tectonic activity in the past as well, but that shut down long ago.

“Each rocky planet has taken a unique path and there isn’t this clear picture of activity where planets take an evolutionary path towards plate tectonics,” says Watters. “It opens a lot of questions.”

Understanding the diverse backgrounds of the planets in our own backyard is an important step to prepare for all the data the Kepler Astronomical Observatory and eventually the James Webb Telescope will bring in on hundreds or thousands of rocky planets found outside our Solar System, says Watters. Now that will be a grand day for astronomy.