How deep is the underwater waterfall?

The shelf it slides off is no deeper than about 150 m; the drop-off it slides into goes down more than 4,000 m. There is no bottom in the sense people mean, because nothing falls. Sand and silt settle down a long slope. What is “at the bottom” is more sand, in the dark, a few kilometres down.

ShelfAbout 150 m at most

Drop-offMore than 4,000 m

What movesSand and silt

What is at the bottomA sand slope

Vertical drop?No — a slope

Why it showsPale shallow vs dark deep

Mauritius sits on a submarine shelf that is nowhere deeper than about 150 m. Inside the reef the lagoon is a few metres; on the shelf outside it the water is tens of metres and reads pale blue from the air over sand. Off the tip of Le Morne the shelf ends and the seabed falls away to more than 4,000 m. That water reads near-black. Currents carry sand off the shelf edge and it slides down the slope, hanging in the water as it goes. The plume of pale sand crossing from pale water to dark water is the waterfall shape.

The depth is the whole reason the illusion exists. Without a hard drop from about 150 m to thousands of metres you would not get the colour contrast, and without the contrast there is no waterfall to see. Where the drop is gentler, and it is gentler around most of the island, the same sand movement is happening and nobody photographs it. Why the eye reads it as falling water is on is the underwater waterfall real.

People ask what is at the bottom as if the sand were going somewhere. It is going down a slope and stopping when the slope flattens, and the ocean floor out there is several kilometres below the surface. There is no cave, no vent, no plughole. Nobody has anything to fear from it and nobody can dive it; the depth that makes it visible also puts it beyond any diver, which is the point of is it safe. You look at it from a helicopter and that is the entire relationship.

Insider tip

The 4,000 m figure surprises people who know the lagoons. Stand on the beach at Le Morne and you are a few hundred metres from water deeper than the mountain behind you is tall, seven times over.

Check dates on the underwater waterfall helicopter flight

Free cancellation up to 24 hours before departure on most tours — book now, decide later.

Frequently asked questions

What is at the bottom of the underwater waterfall?

A slope of sand. The sand slides off a shelf about 150 m deep and settles down the drop-off, which goes below 4,000 m. No cave, no hole, no water falling into anything.

Does the water go down to the bottom of the sea?

No. The water stays put; the sea surface is flat. Only sand and silt move downhill. The mechanism is on is it real.

Can you dive the underwater waterfall?

No. It is open water off the reef over a slope that starts around 150 m and goes to more than 4,000 m. There is nothing to dive on and it is far beyond recreational depth. You see it from a flight; see how to see it.