The mysterious veil on the treasure monument in Petra, Jordan has been lifted again.

Beneath the ancient rock-cut building, archaeologists have discovered a hidden tomb containing twelve relatively well-preserved human skeletons as well as an extensive collection of grave goods.

The discovery comes more than two decades after similar tombs were found on the other side of the famous Treasury, also known as Al Khazneh, which is one of the Seven Wonders of the World and a UNESCO World Heritage Site.

A team of researchers received permission from Jordanian authorities earlier this year to conduct a week of remote sensing surveys in and around the Ministry of Finance, which was the center of a city hand-carved into the walls of a desert canyon by the people of the Nabataean kingdom.

“There was always the thought that there might be more graves, but no one has ever been found that still contained burials,” said Richard Bates, a geophysicist and professor at the University of St. Andrews in Scotland, in an e-mail. Email to NBC News. “The hope was to find an intact grave.”

Using ground-penetrating radar, the joint Jordanian-American team, which also included the Jordanian Ministry of Antiquities and the American Center of Research, a nonprofit organization based in Amman, discovered a cavity and determined its location and depth. Instead of digging straight down, which would have meant cutting through solid rock and damaging part of the building, they carefully hand-digged from the outside toward the cavity, Bates said.

Inside, there were twelve human skeletons in their original burial site, including one that comprised the top of a broken jar, most likely dating to the first century BCE. BC. The remains are most likely both male and female and range in age from children to adults, Bates said, although this has yet to be confirmed.

The discovery could provide new insights into the treasury, whose purpose is still unknown, as well as the Nabataean people.

“Since complete burials have never been found here, it is likely that we will learn much more about the Nabataean kingdom from this find,” Bates said.

The Treasury, which receives more than a million visitors every year, is Petra’s most famous landmark. It appeared in Steven Spielberg’s 1989 film Indiana Jones and the Last Crusade as the cinematic resting place of the Holy Grail.

The excavation of the newly discovered grave was featured in a two-part episode of the American reality television series “Expedition Unknown,” which aired on Discovery Channel.

Bates said there is evidence of other cavities in the area that could also be graves.

“It is very likely that more will be found,” he said, “and that is why we need the funds to continue the studies.”