- [Man] With 71% of our
planet covered with water, it's amazing how little
we actually know about what goes on beneath the waves. New species are constantly being found, but there are much larger forces at work that we're only just
beginning to understand. With that in mind, here are the top 10 strangest
things that happen underwater. (tranquil music) - Amazing! - [Man] Number 10, The Red Tide. Red Tides are an increasingly
common oceanic phenomenon that occurs as a result of
rapidly growing algal blooms. With such large quantities of
red plant matter in a region, the ocean becomes discolored to give the characteristic appearance. It's also potentially very dangerous, with oxygen levels becoming
severely depleted and, in some cases, the release of toxins that are harmful to humans
and other marine life. Some of the potential human illnesses are paralytic shellfish poisoning, diarrhetic shellfish poisoning, and amnesic shellfish poisoning. Red Tides have been
recorded across the world in the Americas, Europe,
Asia, and Australasia, and are becoming more frequent due to warming sea temperatures, low salinities, and increased nutrients being washed into the waters around coastlines, all of which create the perfect conditions for algae to thrive. A similar phenomenon, referred
to as a pink algal bloom, is occasionally seen in colder climates, such as around Greenland. They cause a pink-hued snow,
called watermelon snow, which helps to trap heat on the surface. This actually increases the
melting of the ice sheets because it reduces the
snows' reflectiveness by as much as 10%. Number nine, Undersea Rivers. Off the coast of California, two miles beneath the surface
of the Atlantic Ocean, is the Monterey Canyon. Despite being underwater, it looks quite similar
to the Grand Canyon, and scientists have only
relatively recently begun to understand that this has happened because of a vast undersea river. It's so big that, beyond
the valley itself, it's carved out a route in the ocean floor that extends for more than 200 miles. Phenomena like this are
seen across the world, where these rivers can be many miles wide, and thousands of miles long. The nutrients carried by
these flows sustain life that otherwise wouldn't
be able to survive there, and the geological
features that surround them are similar to what you might
find by a river on land, they meander, have
banks along their sides, and even form oxbow lakes. The difference here, though, is that rather than being flows of water, they're the movements of sediment that are pushed along the ocean's floor. Very little is known about
how this process works, and it's an example of how complicated the oceans of the planet actually are. Number eight, Underwater Crop Circles. Underwater Crop Circles
were first documented by a group of divers off
the coast of Japan in 1995. Soon after the initial find, a number of them were
found in the nearby area. The mysterious thing about
these perfectly arranged geometric formations was
how they often vanished as quickly as they had
appeared in the first place. At first it was a complete mystery how these shapes were occurring, but further study revealed
a surprising answer, it was because of the mating rituals of a species of puffer fish. They were making the
circles to attract a mate. The males swim across the seabed, flapping their fins in a way to disrupt the sand and create the patterns. The fish themselves are
only about 5 inches long, but some of these creations can be as much as 7 feet in diameter. Once complete, the females come to inspect what has been made, and if they like it, they lay their eggs in the center for the males to fertilize. Number seven, The Milky Sea. The milky sea is a place that's been a part of nautical
legends for centuries, and was even mentioned in 20,000 Leagues Under
The Sea by Jules Verne. The stories told of a
large part of the ocean that glowed at night, and while scientists remained
skeptical of the idea because of the sheer numbers of bacteria that would be needed to cause it, researchers found the
world's largest area of bioluminescent ocean in 2005. At 6,000 square miles, the region off the coast of Somalia is twice the size of Connecticut, and is thought to be caused
by a large population of bacteria called Vibrio harveyi. It's not always present, but when it is, it's an incredible site. This isn't the only instance of large areas of bioluminescence
in water though, and there are places that
are far more accessible where you can experience
the phenomenon for yourself. The coasts of Tasmania, for example, are home to colonies of algae that glow under certain circumstances, and so are regions in Hong Kong, Sydney, and even the Indian
River Lagoon in Florida. Number six, Underwater Waterfall. When you think of the biggest
waterfalls in the world, you might think of Niagara
Falls between Canada and the US, or the Victoria Falls
between Zambia and Zimbabwe, but the largest waterfall
to have been discovered is far less noisy and
ferocious than either of these, because it lies underwater
in the Denmark Strait between Iceland and Greenland. We know that cold water is
much denser than warm water, and when different temperature
currents meet in the ocean, the cold water takes
a direct route beneath the warm water in the same ways as water drops on a land based waterfall. In the Denmark Strait, cold water from the Greenland
sea meets the Irminger sea, which causes the colder water
to fall almost 11,500 feet, with more than 175 million
cubic feet of water passing every second. That's the equivalent volume of nearly two thousand Niagara Falls'. Another similar underwater phenomenon is that of underwater waves. Known as 'internal waves', these movements of water can be gigantic, sometimes stretching as wide as 100 miles, and being as tall as a skyscraper. They're super sized versions of the waves we see on the surface, and can be catastrophic for
submarines, drilling platforms, or anything else that gets caught in them. Number five, Naturally Formed Cities. In 2013, Divers off the coast of the Greek island of
Zakynthos made a discovery that many thought would be one of the most significant in recent memory. Near Alikanas bay, about 15 feet beneath the water's surface, they found what appeared
to be paved flooring and the bases of circular columns, which seemed to suggest that these were the remains of a long lost sunken city. What was unusual for this find though, was that there was no
sign of any pottery or any other types of artifacts
that you would expect to see. This piqued the interest of researchers who began to take samples, and eventually came to the conclusion that rather than being a human made city, this was a series of
naturally forming structures. The circular shapes had
actually been formed by mineralisation where hydrocarbons were being released into the
water from a fault within the seabed that hadn't fully ruptured. This happens quite
often in the deep ocean, but is relatively rare
in shallower waters, and results in a release of
gases, primarily methane, which react with the water. Number four, The Great Blue Hole. The Great Blue Hole is a stunning submarine sinkhole on the Lighthouse Reef, which is an atoll in the Caribbean sea that's about 62 miles away from
the capital city of Belize. It's a popular destination for divers, and it's clear to see why. Amidst the turquoise waters is an ominously dark blue circle, which hosts numerous
species such as reef sharks, giant groupers, thousands of tropical fish and amazing colorful formations. It's almost 1,000 feet
across, and is 410 feet deep. The hole was originally a
sinkhole that formed thousands of years ago during the ice age, at a time when ocean
levels were much lower. As water levels rose, it was filled to become what we see today. It's regarded as one of the must-dive locations in the world, where divers will descend into darkness and see massive stalactites. There are actually quite a few similar submarine sinkholes in the world, all forming in a similar way to the Great Blue Hole, which remains the largest ever to be discovered. If Belize is too far to travel, then you can also visit
one in the South China sea, the Bahamas, Australia, or
in the Red Sea in Egypt. Number three, Brine Pools. Brine pools occur deep within the oceans, and are some of the
deadliest places on earth. One such place has recently been found at the bottom of the Gulf of Mexico. Water within the pool has a salt content about 5 times higher
than the sea around it, and once animals enter it, it's virtually impossible
for them to leave. Their decomposing corpses,
along with falling detritus, add toxic chemicals to the water, such as methane and hydrogen sulphide, and make things even more inhospitable. They occur in places where
there's very little water flow, so once material falls in, there's nowhere else for it to go and the chemical levels get
increasingly concentrated. As you approach a brine pool, it looks like a flowing
river within the ocean, but then it becomes apparent
that this is a deadly place, and you'll see corpses
scattered across the floor. The only creatures that are able to survive here are tube worms, some species of shrimp,
and bacterial life, which makes Brine Pools
an area of interest for researchers who investigate how organisms survive in such extremes. Number two, Icy Finger of Death. The ominously named 'Icy Finger of Death' is also known as a Brinicle, and it's a rare phenomenon
that proves deadly for anything that comes into contact with it. It's all possible because when water contains high levels of salt, its freezing point becomes much lower. Brinicles form in both the
Arctic and the Antarctic, where concentrated salty water is released from beneath
larger ice formations. As it is much denser than normal seawater, it falls down towards the ocean floor. This brine is much colder than
the water that surrounds it, because it has trickled through ice, so, as it falls it freezes
the fresh seawater, which forms a tube of
ice around the plume. It essentially becomes an
inside out icicle where, instead of cold air freezing
water from the outside, the sub-zero brine freezes
the water from within. When this brinicle hits the ocean floor, it forms a web of ice that freezes everything it comes into contact with, including any unfortunate
creatures that lie in its path, such as urchins and starfish. Number one, Underwater Black Holes. You may think of them as a
dangerous space phenomenon that are thousands of
light years away from us, but black holes actually
exist in our oceans. These ones aren't collapsed stars, though, but formations that at least resemble a black hole's ability to
prevent anything from escaping. More precisely these are ocean eddies, formed by colliding currents, that create circular movements of water. In the southern Atlantic,
they're said to be to water what a black hole is to light. They are known as maelstroms, and can grow to be much
bigger than a city. They hold billions of tons
worth of water so tightly that nothing can escape from them. These aren't just a
rare occurrence, either, new ones are being discovered every day. By studying them with
satellite observations, scientists have found that
these maelstroms even share the same mathematical
properties as black holes. They prevent water from leaving, and some have been proven to have kept the water molecules within them for years without letting any escape. Weaker maelstroms, with far less power, are much more common, though,
and are usually referred to as whirlpools or water tornadoes. Have you seen any of these yourself? Or are there any that you think
should have been mentioned? Make sure to leave your
comments down below, and thanks for watching! (tranquil music)
