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Belize's Great Blue Hole is one of Earth's most mysterious features.
Anyone peering into the 125 metre (410ft) deep pit will wonder what secrets and stories lie beneath.
Now, scientists have discovered the natural oddity provides evidence of a concerning trend.
A layered sediment core extracted from the bottom of the hole provides a 5,700-year storm archive – and it shows the weather is only getting worse.
In 2022, researchers from Goethe University Frankfurt transported a drilling platform over the open sea to the Blue Hole, and then proceeded to extract a 30-metre sediment core from the underwater cave.
Now, analysis reveals the layered sediment has served as an archive for extreme weather events for thousands of years, including tropical storms and hurricanes.
Dr Dominik Schmitt, a researcher in the Biosedimentology Research Group and the study's lead author, said: 'Due to the unique environmental conditions – including oxygen-free bottom water and several stratified water layers – fine marine sediments could settle largely undisturbed in the Great Blue Hole.'
The sediment core looks a bit like tree rings, with annual layers alternating in colour between grey-green and light green.
The 410ft-deep Blue Hole is the largest in the world and, over thousands of years, has documented weather events through different layers of sediment build-up
The layered sediment acts as an archive for weather events. Different features indicate extreme conditions caused by hurricanes or volcanic eruptions
A man on a bicycle makes his way through a street blocked by branches of fallen trees after the passage of Hurricane Lisa in Ladyville, near Belize City, on November 3, 2022. The researchers said hurricanes like this are likely to be more common this century
Over the years, storm waves and storm surges have transported coarse particles from the nearby atoll reef into the Hole, forming distinct layers at the bottom.
These storm deposits stand out from the fair-weather grey-green sediments in terms of grain size, composition and colour, which ranges from beige to white.
The research team, which also included scientists from Cologne, Göttingen, Hamburg, and Bern, identified a total of 574 storm events over the past 5,700 years.
This provides a much longer snapshot of climate fluctuations and hurricane cycles than instrumental data and human records, which only date back around 175 years.
They discovered that the distribution of storm event layers in the sediment core show the frequency of tropical storms and hurricanes in the southwestern Caribbean has steadily increased over the past 6,000 years.
Higher sea-surface temperatures were also linked to increased storm activity, they found.
Over the past six millennia, between four and 16 tropical storms and hurricanes passed over the Great Blue Hole per 100 years.
However, the nine storm layers from the past 20 years indicate that extreme weather events will be significantly more frequent in this region in the 21st century, the researchers warned.
Two divers investigating the Blue Hole. A sediment core collected from the bottom of the pit provides a 5,700-year storm archive – and it shows the weather is only getting worse
People stand next to fallen trees due to strong winds caused by tropical storm Sara in Tocoa, Honduras, on November 15, 2024
'Our results suggest that some 45 tropical storms and hurricanes could pass over this region in our century alone,' Professor Eberhard Gischler, who also worked on the study, said.
'This would far exceed the natural variability of the past millennia.'
Natural climate fluctuations cannot account for this increase, the researchers warned, pointing instead to the ongoing warming during the Industrial Age, which results in rising ocean temperatures and stronger global La Niña events.
This creates the perfect conditions for frequent storm formation and their rapid intensification, they added.
Writing in the journal Science Advances they said: 'Predictions of tropical cyclone frequencies are hampered by insufficient knowledge of their natural variability in the past.
'A 30m-long sediment core from the Great Blue Hole, a marine sinkhole offshore Belize, provides the longest available, continuous, and annually resolved tropical cyclone frequency record.
'A 21st-century extrapolation suggests an unprecedented increase in tropical cyclone frequency, attributable to the Industrial Age warming.'
10 facts about Belize's Great Blue Hole
1. The Blue Hole was formed at the end of the last Ice Age when rising seawaters flooded a series of enormous caverns.
2. Geologists have determined that the caves first formed about 153,000 years ago and were completely submerged approximately 15,000 years ago.
3. It is located approximately 60 miles offshore from Belize City and is part of the Belize Barrier Reef, a UNESCO World Heritage Site.
4. The underwater caves of the Blue Hole are filled with giant stalactites - proof that it once existed above water.
5. It is home to several shark species including Caribbean reef sharks, nurse sharks, hammerheads, bull sharks, and black tip sharks.
6. While the Great Blue Hole is open on the surface, it features an extensive cave system that requires a lot of diving skill to successfully navigate. Only divers with at least 24 dives under their belt are allowed inside.
7. In 2012, the Discovery Channel ranked the Blue Hole as #1 on its list of 10 Most Amazing Places on Earth.
8. It is visible from space and is easily identifiable by its unique circular formation and location within the midst of the greater Belize Barrier Reef.
9. The name 'Blue Hole' was coined by the British diver Ned Middleton in his book '10 Years Underwater' published in 1988.
10. In 2018 Richard Branson was part of an expedition team who travelled to the bottom of the hole - and found plastic bottles on the sea floor.
