A Florida archaeologist saw shapes in the water. They led to a submerged grave.

From aboard a seaplane in 2016, Josh Marano looked down over the Dry Tortugas and saw something strange. Underneath the clear waters of the coastal Florida national park was a series of dots arranged in an orderly L-shape. The pattern didn’t look natural, he thought.

He put aside the hunch until years later. Finally, in August, Marano assembled a small research team to dive to the site and investigate what he’d seen. The dots turned out to be old building foundations that stood on a now-submerged island. Near them was a slab of sandstone, covered in seaweed and inscribed with gothic lettering: “John Greer” and “Nov. 5. 1861.”

Marano had discovered a Civil War-era tombstone, buried in the Gulf of Mexico off the coast of Key West. With it came a mystery: Who was Greer, and how did he die?

“It was surreal,” Marano said. “It’s the equivalent of finding a bell on a shipwreck with the name on it. It’s just something that you absolutely never hear of in underwater archaeology.”

The National Park Service announced the discovery on May 1 after Marano and his team completed additional research about Greer and his burial site. They determined through historical records that Greer, a civilian laborer, was buried on the submerged island near the Dry Tortugas’ landmark fortress, Fort Jefferson. The island once housed a quarantine hospital and a cemetery that probably still contains the remains of dozens of American service members.

On a group of islands known for snorkeling, shipwrecks and military history, Greer’s headstone is a link to the overshadowed stories of workers - both employed and enslaved - who built Fort Jefferson and once called the Dry Tortugas home, Marano said.

“It kind of opens the door to a lesser-documented history that we’re hoping to highlight,” Marano said.

Sunken treasures are essential to the mythos of the Dry Tortugas. The string of islands was a hub of piracy in the 17th and 18th centuries, and the seabed is strewn with shipwrecks from treasure fleets.

But there was no gold or silver to plunder when Marano and a small team of researchers dived into the park’s waters in August to investigate his observation from years earlier. What they found instead harked back to a subsequent chapter in the Dry Tortugas’ history, when the U.S. military built a strong presence on the islands to better defend the Gulf of Mexico.

In 1846, construction began on a sprawling brick fortress, Fort Jefferson, that remains today as the national park’s landmark attraction. During the Civil War, Fort Jefferson became a prison that most famously housed Samuel Mudd, a doctor convicted of conspiracy in the assassination of Abraham Lincoln.

Other structures were built on the islands surrounding Fort Jefferson to complement the growing population on the Tortugas, according to the National Park Service. Before beginning their dive, Marano and his team determined through historical records that his observation corresponded with the location of a submerged island that once contained a hospital.

Marano’s team first found pylons jutting out from the sand, just beneath the water’s surface. They were about to swim away from the site when Devon Fogarty, an archaeology student on Marano’s team, spotted a slab of sandstone nearby. Marano cut away several strands of seaweed growing from the rock and revealed the lettering underneath. The team was shocked.

“We had no idea, like there was no inkling,” Fogarty said. “We knew that there are cemeteries out there, that there are grave plots, but we didn’t expect anything to be preserved.”

Sand islands in the Dry Tortugas can grow, shrink and shift in location as they’re buffeted by the elements, Marano said. But the complete submerging of an island is rarer. The island’s disappearance was accelerated by climate change and major storms that have swept through the region, the National Park Service said.

Marano’s team determined that Greer’s grave was part of a cemetery on the island for service members stationed at Fort Jefferson and for civilians. But they didn’t want to announce news of the discovery before learning more about Greer and his life, Marano said. So he and Fogarty embarked on a new hunt: an arduous search through reams of archived records to find Greer’s name and any hint of the person he was.

They struck out at several archives in Florida. In November, Fogarty traveled to the National Archives in Washington to try again. There, she finally made a breakthrough: Greer was listed as a White laborer in 1857 who’d built scaffolding during the fort’s construction. That hinted at the hardships that could have led to his death, Fogarty said. She theorized - based on the fact that Greer did not appear in hospital records - that he could have died in a sudden accident while on the job.

“He was working in a very dangerous occupation, building very unsafe scaffolding to take up all those construction materials,” Fogarty said.

Marano and Fogarty were unsuccessful in attempts to find descendants of Greer, they said, though they hope news of the discovery will encourage any relatives of people who’d lived on the islands to share any personal records from the period.

The team plans to continue research on the submerged island, which is now recognized as a federally protected archaeological site, Marano said. Questions remain about the nature of Greer’s death, the lives of those buried with him and the numerous enslaved laborers who also helped build Fort Jefferson.

Marano and Fogarty hope to answer them. They see it as a reversal of sorts for the history of a fort named after Thomas Jefferson.

“We want to be able to establish that we can put the extra effort into remembering regular people,” Fogarty said.