Blue holes show hurricane activity in the Bahamas is at a centuries long low

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The 2021 Atlantic hurricane season was the third most active in 171 years of record-keeping. The 2020 season was even worse—there were so many tropical cyclones that meteorologists tore through their list of possible storm names and needed the Greek alphabet to keep track of the overflow. But a new study shows that even this flurry of activity may be something of a lull in the centuries-long record of Atlantic hurricanes.

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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. These open pits form in limestone, often above collapsed caverns. Prolonged erosion weathers the edges into an eerily circular shape.

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The seafloor at the base of a blue hole acts like a calendar of past storms. Much like an ice core or tree ring grows season after season, the sediment at the bottom of a blue hole builds up over time. Natural currents coax a sugary sprinkle of small sand grains into the hole, while violent hurricanes pitch larger grains into the pit. By comparing layers of coarse and fine grains in this sedimentary lasagna, researchers can count how many hurricanes passed nearby. What makes a blue hole a valuable long-term record is that once this sediment settles, there’s very little activity in the pit to disturb it.

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