In the depths of the Pacific Ocean, a hydrothermal vent that the remotely operated vehicle (ROV) named SuBastian explored in early March is a remarkable sight to see. At a depth devoid of light, creatures and microbes thrive off of the chemicals spewing from deep inside the Earth. Over time the chemicals accumulate around the vent to build mineral towers, some upwards of 60 feet tall (20 meters), like upside down stalactites spotting the landscape. But the intense heat also causes an optical illusion, one that even leaves seasoned scientists with feelings of awe. Six thousand five hundred feet (2,000 meters) beneath the sea, a shimmering surface reveals a world never before seen.

“The immense beauty and majesty of the scene was overwhelming. It is something I will never forget,” says Mandy Joye, a professor at the University of Georgia and the lead scientist of the Schmidt Ocean Institute expedition that encountered the phenomena.

The vent Joye and her team explored is part of a system within the Gulf of California, specifically in a depression called the Guaymas Basin. The mission to explore and collect specimens from the hydrothermal vent system there allows scientists to learn more about this still mysterious ecosystem—the first visual confirmation of a hydrothermal vent ecosystem was in 1977. Scooping up a sample of soil may hold hundreds of new microbe species, some of which could reveal insights into how life first began on our planet. For a microbiologist, it is like being a kid in a candy store. Yet five hours into this mission, the scientists were enthralled by an optical illusion, and not a single sample had been collected.

As the researchers watched the video stream from SuBastion 2,000 meters below, along with a live online audience, they were in awe of what they saw. At first glance, the underside of the rock overhang looks to be a sheer and level surface, so smooth that the tube worms and rock formations below appear in a perfect reflection. But a slight change in the ROV’s angle and suddenly our perceived reality shatters. The mirror surface disappears and behind it is a cavernous arc of glittering minerals that twinkle like the night’s sky. What exactly are we looking at?

The magic of the looking glass comes down to the nature of the hydrothermal vent. The water seeping from the vent is extremely hot, as the pressure of the ocean depths allows the water to reach temperatures of about 690 degrees Fahrenheit (366 degrees Celsius) without boiling. Hot water, less dense than cold water, rises as it mixes with the surrounding ocean. But here on its journey upward it comes in contact with the jutting rock formation, what scientists call a “flange.” Trapped with nowhere to go, the water fills the concave overhang. The temperature difference between the trapped water and the cool surrounding water is so great that it causes light to slow as it travels from the cool to the hot water, creating a false surface that appears with the shine of a mirror.

“Think of air versus water—light slows down in water so you can see your reflection on the surface of a lake if you look along the correct angle. [At a different angle], you can look through the surface beneath the water. The same thing happens here,” Joye says.

As for the glittering cavern, scientists don’t yet know what causes the sparkle, but it is likely some sort of mineral. One possibility is pyrite, often called “fool’s gold,” a mineral of iron and sulfide. Another is pyrrhotite, a common mineral that forms when vent water mixes with the surrounding seawater. Only after a piece from the cave is examined in the lab back on shore will scientists be able to determine what causes the glittering in the rock formation.

According to Joye, this mirror illusion has been seen only once before, at another location within the Gulf of California, but the size pales in comparison to this new find in the Guaymas Basin.

“A discovery like this is an incredible thing to be a part of. I could not believe my eyes. It was just surreal,” Joye says.