With its striking red sandstone buildings, the city of Petra in Jordan is one of the world’s most famous ancient wonders. Experts are still learning about the city to this day, which was founded sometime during the mid 2nd century BCE by an Arab tribe called the Nabateans, abandoned during the 7th century CE, and rediscovered by explorer Johann Burckhardt in 1812.

According to National Geographic, archaeologists recently discovered a massive monument at Petra that eluded other diggers for years. It sat in plain sight, half a mile south from the city’s center—but it took Google Earth, high-resolution satellite imagery, and aerial drone photography to spot it. The find was recently published in the journal Bulletin of the American Schools of Oriental Research.

The building, NatGeo reports, is as long as an Olympic-size swimming pool and twice as wide. It consists of several parts: a large, 184-by-161-foot platform base surrounding a smaller, flagstone-paved platform on which a large, east-facing staircase crowned by a row of columns once stood. A 28-square-foot structure, which sits atop the interior platform, once faced the stairs.

Excavators found pottery dating from the mid 2nd century BCE, indicating that the structure was built during Petra’s early years, as the Nabataeans first started constructing public buildings.

"We know it's large, it's significant, it's important. It probably would have had some kind of a public function,” archaeologist Sarah Parcak told NPR. She discovered the structure along with Christopher Tuttle, executive director of the Council of American Overseas Research Centers. “Could it be used for religious purposes? Was it some sort of public administrative structure? I wish I knew."

Parcak is a prominent “space archaeologist” who uses satellite images taken by cameras hundreds of miles above Earth to find lost sites. She’s used this advanced technology to search the Canadian coastline for potential Viking sites, and to scour Egypt for lost cities, temples, and tombs. This time around, she wanted to look for potential new features at Petra.

Parcak originally dismissed satellite imagery of the large structure, but she informed Tuttle, who was conducting groundwork at Petra, about the find. He ended up finding the site’s foundations and column bases.

The monument's existence surprised Parcak, who told NPR that she “thought that maybe we'd find some small stone structures or roads, but we didn't think at all that we would find anything large just because Petra is a World Heritage Site and it's been worked on intensively for nearly 200 years.”

Tuttle, who's investigated Petra for 20 years, was a little less shocked. “I knew that something was there, but it's certainly legitimate to call this a discovery,” he told National Geographic. 

[h/t National Geographic]