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1. The Baltic Sea Anomaly

The eerie 'Bloop’ was once thought to originate from a whale

In 2011, a group of divers looking for treasure and salvage in the Baltic Sea took a sonar image of a 70-metre-long and steely-looking object lying 300 feet below sea level. Some claimed that ‘The Baltic Sea Anomaly’, as it came to be known, was a UFO, its resemblance to the Millennium Falcon in Star Wars fuelling that speculation. Others said it was the remains of a lost city. When scientists looked further into the anomaly, they came to the conclusion that it was a huge glacial deposit left over from the thawing glaciers that carved out much of the Baltic Sea basin.

2. The Bloop

It sounds like a horror movie monster, but ‘The Bloop’ was actually an ultra-low-frequency, high amplitude underwater sound. Detected by the US National Oceanic and Atmospheric Administration (NOAA) in 1997, the sound was loud, unprecedented and eerie. Explanations offered included covert underwater military exercises, whales, giant squids or a totally unknown sea creature. However, it seems glaciers were once again the culprit, with the sound judged to be consistent with ice calving, where large chunks of ice break off a glacier.

Measuring up to six metres across and sometimes decorated with shells and coral fragments, these ornate circles were discovered on the seabed near southern Japan in 1995 and remained a mystery for 16 years. As with crop circles on land, alien interference was cited as an explanation. However, in 2011, researchers discovered that it was the male pufferfish – at just five inches long – that was building them. The pufferfish were observed to spend days swimming in and out of the circle area, using their fins to dig into the sand. What was all the effort for? To attract a mate!

The narwhal is another puzzling creature form the depths, and the function of its long tusk has been the subject of much debate. Many researchers believe it is used for sexual seduction, much like antlers on a deer. Other theories suggest the tusk is a sensory organ able to detect changes in the environment or a weapon for doing battle with other narwhals and predators. In 2017, drone footage recorded by Canadian scientists suggested that Narwhals can use their tusk to stun fish while hunting.

6. The Mary Celeste

A ship like the infamous Mary Celeste

The Mary Celeste has long been a byword for eerie and deserted. That was exactly the scene found by Canadian sailors when they boarded the American vessel off the coast of the Azores in December 1872. The crew’s belongings and the ship’s cargo were discovered intact, but the lifeboat was missing, as were all of the 10 people who were recorded as on board. The last entry in the ship’s log was written over a week before it was found, but it shed little light on why the Mary Celeste never completed its journey from New York to Genoa.

7. The Bermuda Triangle

Like The Mary Celeste, the Bermuda Triangle has become synonymous with intrigue and unexplained disappearances. The triangle is an area of 500,000 square miles of ocean between Miami, Bermuda and Puerto Rico. The military ship the USS Cyclops, travelling from Baltimore to Brazil with 300 crew, went missing there in 1918. In 1945, a whole squadron of bombers similarly vanished. In both cases no wreckages were found.

8. The unmapped ocean floor

Seventy percent of the Earth’s surface is below the ocean, but although the ocean has been mapped, only large details (of three miles or more in length) can be properly seen – this means that 95% of our oceans remain hidden. It’s little wonder, then, that the oceans continue to throw up surprises. Recent discoveries have included a new shark (Genie’s dogfish) and perhaps the longest animal ever recorded – a 47-metre-long Apolemia (jellyfish), discovered earlier this year by Australian scientists.