The evidence that hurricane activity is at a historical low is hiding on the Caribbean seafloor, tucked away in odd geological features called blue holes. Blue holes are similar to sinkholes but on a much grander scale. They can be 300 meters deep, like the Dragon Hole in the South China Sea, or 300 meters wide, like the aptly named Great Blue Hole in Belize. The Bahamas is home to the world’s greatest concentration of blue holes, making it an appealing destination for paleotempestologists—scientists who study historical tropical cyclone activity. The seafloor at the base of a blue hole acts...