Unbelievable but true – Underwater waterfall in Mauritius!

Bluish sails boat going off sea cliff on right and water falling on two sides into a crack on left at the end of Mauritius forming an underwater waterfall; text 'wouldn't it be amazing if this venture of yours took you to the world beyond this one? Pirates of the Caribbean - at world's end' written on picture.

Ever heard of an underwater waterfall? The 7 cascades, Rochester falls, are common waterfalls that we are used to since they are on land and are easily accessible. Sounds pretty strange too, water falling under water! Yet, this amazing phenomenon does occur in Mauritius.

Where is the underwater waterfall found?

This amazing water drop-off is found in the south west of the island, at Le Morne and faces the Brabant Mountain. It is an important area of the island and attracts tourists and islanders alike as it forms part of the UNESCO World Heritage Site. This is where slaves used to jump off the cliff to embrace death back in the 1800s. However, the beautiful waterfall cannot be seen from land nor from the mountaintop; it is only visible above the sea, in helicopter, plane or by drone.

How can a waterfall be underwater?

The term itself means water-fall, as in water falling from an upper region to a lower one, thus forming a water gradient. Is it possible for water to fall underwater when the underwater is already water? Tricky, but yes, very possible.

Now, what exactly is a waterfall? Put simply, a waterfall is water falling from high grounds to low grounds, hence waterfall.

The ocean is made up of two different layers of water: warm, light water and cold, dense water. Water in the upper surfaces of the ocean evaporates quickly and so becomes lighter as its density lowers. Since light does not go beyond 1000 metres underneath the sea, the water in those layers is mostly cold and heavy and tends to sink to the bottom. The ocean currents ensure the movement of these two water bodies.

As surface water heats, it becomes lighter and evaporates. Tides and wave action ensure mixing of upper and lower layers thus cold water is pushed upwards but sinks because it is heavy. When this heavy load of water knocks against a hard body such as elevated seafloor it follows its course just as water slides on the floor when it falls out of a bottle. The density of this water prevents it from mixing with the warm layers like oil floats on water.

Using this principle, an underwater waterfall can indeed form. Dense water from atop an oceanic cliff can fall down following the pathway of the so-named feature. It occurs mostly in areas where huge amounts of fresh water, such as snow, melts and dives into the sea.

Underwater waterfall formation: dense cold water from a cliff flowing down the rugged brown seafloor in a separate layer from the light warm water.
Formation of an underwater waterfall

So, how did the underwater waterfall in Mauritius form?

Mauritius island was formed by a shield volcano 10 million years ago. To understand how the underwater waterfall was formed, it is important to understand the geology of the island. In the west, north-west and south-west part of Mauritius, there is a long rift zone that extends from the tip of Le Morne up to Grand Baie. A rift zone is a series of cracks along the volcanic flanks through which magma emerge.

The zone where water appears to be cascading into a crevice at Le Morne is actually a fault in the volcanic shield of Mauritius. Some 18,000 years ago, sea levels were lower than what they are now; sediment analysis in the south-west region shows that the lagoon now formed was once exposed. It is on this mud bed that a system of coral reef, stretching over a distance of 5 km, emerged [1]. Beyond the reef, the slopes of the volcano go into deep waters. Active weathering and rock fall over the years softened the ground and eventually water movement helped in deepening the crack.

Just like the seabed is covered with sand and sediment, the regions around the crack are also covered with the same materials. Water currents constantly move from the ocean towards the coast and vice versa thus causing the sand to slip into the crack. It is this downward slip of sand into the void that causes an optical illusion of a waterfall.

In fact, there is no water falling at all in this scenario. Since the crack is underwater, it is already filled with water to the same level as the flat surfaces around it.

On a more realistic point, the optical illusion of waterfall is not like falling off the flat surface at the end of the world. Most of the images are tricked for a more dramatic view. The actual waterfall does exist as per Google map but can mostly be noticed from above by the colour change in the area from green to blue indicating a drop in the sea floor and the flow of sand downhill around the region.

Google map indicating the location of the underwater waterfall in Mauritius.
Underwater waterfall in Mauritius

Do underwater waterfalls really exist?

Emphatically yes. In many regions around the world such as the Bahamas, divers have reported the phenomenon. Strangely, the largest waterfall in the world is underwater! The Denmark Strait, found in Denmark in between Greenland and Iceland, measures 11,500 feet around 4 times the size of the biggest waterfall on land.


Reference:

  1. Saddul, P. (2002). Mauritius, A Geomorphological Analysis. Moka, Mauritius: MGI, pp 79

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