In 1721, pirates attacked and seized a Portuguese ship carrying a massive trove of treasure en route to Lisbon. Now, researchers believe they’ve discovered its remains off the coast of Madagascar.

The discovery comes from two researchers from the Center for Historic Shipwreck Preservation in Massachusetts, who have conducted several studies on the wreckage over the last 16 years. They say new clues have revealed the ship’s identity as the Nossa Senhora do Cabo, a 700-ton warship. Their findings were published in Wreckwatch Magazine, but have not been peer-reviewed.

The wreck lies near the shores of Nosy Boraha, an island off Madagascar’s northern coast historically known as Île Sainte-Marie, a notorious pirate stronghold during the “Golden Age of Piracy.” The capture of Nossa Senhora do Cabo was “among the most dramatic episodes” of this era, the authors wrote in the study.

In the study, the researchers conducted underwater excavations using sonar imaging and remote sensing technology. They identified the ship based on its structure and artifacts recovered from the ship’s remains, along with archaeological records. They discovered religious figurines and objects made of wood and ivory, including one depicting the Virgin Mary, part of a crucifix, and an ivory plaque bearing a religious inscription. According to the researchers, these items were likely produced in Goa—which, at the time, was colonized by Portugal—and were being transported to Lisbon.

Archeological records indicate the Nossa Senhora do Cabo carried gold and silver bars, coins, silks, and an “extraordinary” array of precious stones, including 110 diamonds, 250 emeralds, 20 rubies, and 20 sapphires, the authors wrote in the study. Study authors Brandon Clifford, co-founder and director of the Center for Historic Shipwreck Preservation, and Mark Agostini, an archaeologist at Brown University, described the entire haul as “an eyewatering treasure, even by pirate standards,” estimating the cargo’s value at over $138 million in today’s money, Live Science reported.

Records also show that the Nossa Senhora do Cabo departed Goa in 1721, bound for Lisbon with a Portuguese viceroy, the Archbishop of Goa, and more than 200 enslaved people from Mozambique aboard. The vessel was attacked and captured by pirate ships on April 8, 1721, near the French island of Réunion, and later taken to Île Sainte-Marie, where it was ultimately scuttled. Among the pirates that seized the ship was the infamous Olivier “The Buzzard” Levasseur.

The Portuguese ship had already been badly damaged in a storm and offered almost no resistance, the authors wrote. While the viceroy was eventually ransomed, the fate of the archbishop and the enslaved individuals remains unknown.

Clifford told Live Science that many shipwrecks from this era were deliberately sunk or otherwise lost nearby, with estimates suggesting that between seven and ten wrecks lie around Île Sainte-Marie alone. Agostini added that the site has historically been overlooked by researchers, and so there’s ample room for more discoveries that give us a glimpse into the past.