Scientists are gearing up to dive into a deep, dark “blue hole,” a rare type of sinkhole on the bottom of the ocean, off the coast of Florida next month.
The research expedition into “Green Banana,” the name of the hole, will reveal new insights about these otherworldly chasms in the seafloor, such as the nature of their inhabitants and their potential relationship to groundwater aquifers. It follows another expedition in 2019 that probed the depths of a blue hole called Amberjack.
“This will be the first time that we get to characterize Green Banana, so we are very eager to see how that hole might compare (chemically and biologically) to Amberjack,” said expedition member Emily Hall, a staff scientist and program manager at Mote Marine Laboratory, in an email.
“It's a different shape and it's deeper, so we aren't sure what we'll find!” she added. “That's one of the great things about exploration…it'll all be new and exciting!”
Blue holes are found around the world and can extend nearly 1,000 feet deep, which is the approximate depth of Dragon Hole in the South China Sea. Needless to say, diving into these gaping undersea voids can be extremely dangerous—in fact, the Red Sea Blue Hole in Egypt is among the deadliest diving sites in the world.
That said, these holes are unique biodiverse habitats that may contain corals, mollusks, sea turtles, or even sharks at any given point in time. They also provide nourishment to adjacent ecosystems, earning them a reputation as the “oases” of the sea and making them tantalizing destinations for scientific research.
Hall is part of a team of scientists based at Mote Marine Laboratory, Florida Atlantic University/Harbor Branch, Georgia Institute of Technology, and the U.S. Geological Society. With the support of NOAA’s Office of Ocean Exploration and Research, the researchers have begun to probe blue holes in Floridian waters.
“We believe that they are likely old springs and sinkholes that formed maybe 8,000+ years ago when Florida was much wider,” Hall said of these holes, though she added that there’s much more to be learned about the origins and evolution of the caverns.
“Each hole that we have studied so far have had differences, chemical and biological,” she noted.
The team dove into Amberjack Hole, located about 30 miles west of Sarasota in the Gulf of Mexico, in May and September of last year. The divers dropped more than 100 feet under the ocean’s surface before they even reached the rim of the hole, which is itself about 237 feet deep.
Expedition member Jim Culter, a senior scientist at Mote Marine Laboratory, described the descent into Amberjack as “a contrast in extremes” in an email.
“At the surface the water is very warm, +85°F and usually clear (in summer),” he said. “As you descend the water begins to cool and a thermocline (abrupt temperature change) usually occurs at 70 to 90 feet deep and the visibility is often somewhat reduced.”
“At the rim of the hole is where most of the action occurs, fish, turtles, crustaceans, small corals, sponges,” he continued. “Going over the edge and a bit deeper the water gets cooler and light begins to fade. A second thermocline occurs around 130 to 150 feet and the temperature is in the mid 60s °F.”
At that point, he said, large marine species like corals and fish are replaced by bacterial clouds and the “rotten egg” smell of hydrogen sulfide.
“The bottom is soft, very silty and easily stirred up,” Culter said. “It is dark until you look up and see the round opening of the hole framing the blue water 150 feet above.”
Hall remained close to the opening of the hole during these dives because her SCUBA training limits her to depths of 200 feet (which is still extremely deep!). However, she noted that “the wildlife is booming” even at the rim of the hole.
“You get a little bit of an adrenaline rush, and more and more questions start running through your head…what's down here?…where does this lead to?…can I go just a little bit deeper?!” Hall recalled. “Most of the time we are working, collecting water samples, sediment samples, doing diversity counts, but sometimes, just for a second or two, we do get to just ‘look’ in awe!”
In addition to widely sampling the habitat, the team discovered intact carcasses of two smalltooth sawfish at the bottom and brought one of them—a 12-foot-long male—back to the laboratory to study.
These natural marine graveyards are constantly fed by the dead things that float down from above, but the researchers also recorded nutrients emerging from Amberjack Hole, indicating that the holes supply nourishment to the marine ecosystem outside.
“We are seeing much higher concentrations of nutrients in these holes compared to the surrounding water,” Hall said. The source of these nutrients is still unclear, though they may emerge from activity in the bottom sediments, or as a result of tidal flows or links to a larger aquifer network.
Hall and her colleagues hope to constrain some of these juicy questions during their expeditions into the Green Banana, which is about 425 feet deep from bottom to rim, and more than 150 feet under the sea surface. Nobody is certain of the origins of the name Green Banana, and the hole goes by other names too, but Culter said the rumor is that locals named it after a bunch of green bananas that happened to be floating nearby.
The team is set to explore the hole during dives in August and in May 2021, and they hope to find and probe more of these strange structures in the coming years.
“We deal only with holes that are offshore, generally 20 or more miles from shore,” Culter said. “With that in mind there are about 20 documented holes, which are sites we have [been to] personally or have reliable reports.”