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Researchers in Florida are about to dive 425 feet into an offshore sinkhole

Dubbed blue holes, the formations teem with marine life. But scientists know little else about them. What will they find?

It may have been about 10,000 years ago on a wider Florida, land extending 100 miles into what is now the Gulf of Mexico, when a section of the ground crumbled.

Slightly acidic water had trickled through limestone, dissolving the rock and opening a modern horror: the sinkhole.

Over millennia, ice sheets melted and seas expanded. The gulf overtook the hole. By the time commercial anglers came to know it as a hot spot for fish in the 1970s, the rim was 150 feet below the waves. Some captain spotted the peel of an unripened banana floating past, and long after the chasm opened, it got a name:

Green Banana.

At least that’s the story people tell. The truth is, to this day, scientists know little about the hole, about 50 miles southwest of Sarasota. It is one of several believed to exist in the Gulf of Mexico.

The knowledge gap could close some next month when a team of researchers from Mote Marine Laboratory, Georgia Institute of Technology and several other institutions begin studying Green Banana (the coronavirus permitting) up close. Analysts will send equipment to its floor, about 425 feet down.

“They’ve been known for a long time, just never described really,” said Jim Culter, a senior scientist at Mote who has dived to offshore holes, including Green Banana, for decades. “It’s totally an exploration project because we don’t know the answers. Are they valuable, necessary habitat? Do they affect the area around them, or are they just these isolated areas?”

Culter’s hunch is the holes matter more than people realize. Scientists believe they might connect to the Floridan aquifer, and some wonder whether they could contribute to saltwater intrusion into the drinking supply. When Culter first learned of them, he said, the rumor was holes were freshwater springs (several look just like them), but researchers have not found evidence of active flows today.

Dubbed blue holes, the formations teem with marine life around their rims, said Emily Hall, a staff scientist at Mote. That’s one reason she is eager to better understand their chemistry, and it’s also why the locations of holes aren’t always well-known. They are typically identified by commercial fishermen, Culter said, who are not exactly loose-lipped with the coordinates for their honey holes.

Capt. Larry Borden, who at 73 years old is still spearfishing out of Bradenton, took divers out to holes years ago. The Green Banana, he said, had already been heavily fished by the time he found it. It’s not even his favorite hole. That distinction, Borden said, is for a place called “Awesome Spring,” which he named decades ago when he dove down and came up shouting.

“It was fish solid all the way from the surface to the bottom,” Borden recalled. Red snapper, amberjack, “I’ve never seen a spot that has more fish.”

In the years since, Borden said, some holes have been fished a lot and are less lively than he remembers. He laments that there was not funding to study them earlier, especially to determine whether they are linked to the aquifer.

The Green Banana expedition is supported by a federal grant for ocean exploration that depends on technology, Hall said. Previously, the research team explored a place called Amberjack Hole, shallower and closer to shore.

On the bottom, they discovered two intact carcasses of endangered smalltooth sawfish. The holes, Hall said, are replete with nutrients and carbon, which could come from microbes that flourish around them. She wants to learn more about whether the nutrient load contributes to algal blooms in the Gulf of Mexico.

Inside the holes, she said, the pH level drops, offering a test case of what water might be like and which species would thrive after ocean acidification, which scientists say could continue as people add carbon dioxide to the atmosphere. She wonders whether the holes are connected. Do animals swim from one to another, or is each its own special zone?

The project relies on what researchers call a Benthic lander, a boxy device that weighs hundreds of pounds and is loaded with scientific instruments. The crew lowers the lander and anchors it to the floor of the hole. Technical divers plunge to shepherd the device.

Amberjack Hole, Hall said, had an opening about 75 feet wide, whereas Green Banana poses a new challenge because one point in its descent is only about 20 or 30 feet across.

The research team will depart early each morning for a week from Mote to chug out to Green Banana, where they will take samples of the water and sediment and hope seas stay calm enough to drop the lander. The project was supposed to go forward this spring, but threats of the pandemic and travel restrictions forced a delay.

The work is challenging especially for divers, Culter said. Extreme depths demand careful planning and ample time for people to surface without getting sick from the bends.

The exhilaration, Culter said, comes from “finding things that no one else has known about or looked at.”

“The only time I saw whale sharks in the Gulf of Mexico has been over one of these blue holes while we were in it,” Culter said. He has spotted barracudas and sea turtles in abundance.

Swimming deeper requires only a gentle kick of the feet. “It’s kind of like slow-motion flying into the bottom,” Culter said “You’re totally isolated, and you’re totally reliant on your technology to keep you alive.”

The work has inspired Hall to train to dive further herself. From the boat deck, she said, you can’t see anything.

“You dive down and all of a sudden this hole opens up in front of you, and you’re like, ‘Oh my God, there’s this abyss in the middle of the Gulf of Mexico?‘ "

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