An 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.”