About 87 million miles (140 million kilometers) above the Grand Canyon, an even larger, grander abyss cuts through the gut of the Red Planet. Known as Valles Marineris, this system of deep, vast canyons runs more than 2,500 miles (4,000 km) along the Martian equator, spanning nearly a quarter of the planet’s circumference. This gash in the bedrock of Mars is nearly 10 times as long as Earth’s Grand Canyon and three times deeper, making it the single largest canyon in the solar system — and, according to ongoing research from the University of Arizona (UA) in Tucson, one of the most mysterious.

Using an incredibly high-resolution camera called HiRISE (short for High Resolution Imaging Science Experiment) aboard the Mars Reconnaissance Orbiter, UA scientists have been taking close-up shots of the planet’s strangest features since 2006. Despite some truly breathtaking images of Valles Marineris — like the one below, posted to the HiRISE website on Dec. 26, 2020 — scientists still aren’t sure how the gargantuan canyon complex formed.

The Tithonium Chasma (part of Mars’ Valles Marineris) is slashed with diagonal lines of sediment that could indicate ancient cycles of freezing and melting. (Image credit: NASA/JPL/UArizona)