Given that around 70% of our planet is covered in water, it's hardly surprising that untold mysteries and amazing phenomena have been discovered beneath the waves, from ancient shipwrecks to thriving ecosystems.
The modern age of deep-sea exploration dates back to the late 1700s when French scientist Pierre Simon de Laplace calculated the depth of the Atlantic by measuring tidal motions on the Brazilian and African coasts, but it took mankind several centuries to truly conquer the oceans, and even now much of their depths remain uncharted.
In 1960, a research bathyscaphe called the Trieste became the first manned vessel to reach the bottom of the Challenger Deep in the Pacific Ocean's Mariana Trench near Guam, the deepest known point in the Earth's seabed hydrosphere.
This was a milestone for deep-sea exploration, but the discoveries haven't stopped there, with researchers having stumbled upon all kinds of weird and wonderful phenomena, not just in our oceans, but the depths of our rivers and lakes too.
From mysterious stone structures, to lifeforms that don't play by the usual rules, here are the most bizarre and mysterious things ever found during deep dives.
Honourable Mention: A Sunken Sculpture Park
Though not strictly unexplained (and hence why it's included as an honourable mention only), this entry was just too intriguing not to include.
The deep-sea gallery of sculptures in the Molinere Beausejour Marine Protected Area of the Caribbean Sea, off the coast of Grenada, gets an honourable mention for inspiring no less awe than some of mankind's more mysterious aquatic discoveries.
Located between four and five metres deep, the collection of statues is designed to enhance a conserved section of the ocean and highlight the way nature makes a permanent mark on the physical world over time.
Many of the sculptures, which includes life-sized casts of local children, are covered in sea vegetation and barnacles, and have been a hit with scuba enthusiasts for years.
The artwork has also encouraged marine life to take up residence in this stretch of the Caribbean Sea and thus proved beneficial to the environment.