Key takeaways
- The lagoon is the sheltered water between the shore and a coral reef standing offshore.
- Pale over shallow sand, turquoise over a few metres, olive over coral, navy past the reef.
- The reef stands far enough offshore here to enclose the biggest continuous lagoon on the island.
- The whole flight is spent following those bands, which is most of what the eye does up there.
What a lagoon actually is
Mauritius is a volcanic island, and like most tropical volcanic islands it is ringed by coral. The coral grows where the conditions suit it, which is in warm, clear, shallow water with reasonable wave energy, and over a very long period it has built a reef that stands offshore rather than against the shore.
That reef takes the swell. Waves arriving from the open Indian Ocean break on it in a continuous white line, and the water behind it is left alone. What sits behind is the lagoon: shallow, sheltered, warm, and floored largely with pale sand made from ground-up coral and shell.
The distance between the reef and the shore is not the same all the way round. On the east it is generous, and the result is the largest continuous lagoon in Mauritius. That is the water this flight is spent above.
Why it is that colour
Water absorbs light unevenly. Red and orange are taken out within the first few metres; blue and green travel much further. So the colour you see depends almost entirely on how far the light has travelled through water and what it bounced off on the way back.
Over a metre of water on white sand, almost all the light returns and the sea reads nearly white with a green cast. Over two to four metres it reads as the turquoise that everybody photographs, because the water has taken enough red out to tint it but the sand is still returning plenty of light. Over a coral flat, where the bottom is dark and textured rather than pale and smooth, it goes olive and mottled. And past the reef, where the bottom is hundreds of metres down and nothing is coming back, it goes abruptly navy.
That is why the lagoon looks like a map of its own depth. The colour is not decoration; it is data. From the air you can read the bathymetry directly, which is why sandbanks, channels and coral heads are all obvious from a seat in a light aircraft and completely invisible from a boat crossing the same water.
Colour bands over the reef, from pale sand to deep water
What you can pick out
The sandbanks are the first thing. They appear as pale shapes, sometimes breaking the surface as small beaches, sometimes lying just under it, and they curve in the direction the current has been pushing them for a very long time.
The channels are the second. Deeper lanes cut through the shallows, reading as darker lines, and boats follow them because they have to. Watching a speedboat trace a channel from above is one of the small pleasures of the flight, partly because the boat clearly knows something you would never have guessed at from sea level.
Then the coral: individual heads scattered under the shallows like a texture, and larger flats that read as continuous dark ground. Where the coral rises close to the surface the water above it lightens again, so the flats have their own internal pattern.
The reef line
And finally the reef line itself, standing offshore as a white thread of breaking surf. From the beach that is a distant sound. From the air it is a boundary, and everything about the lagoon makes sense once you can see it.
Everything about the lagoon is downstream of that line. It decides where the sheltered water ends, it decides where the sand comes from, and it decides where the shelf edge lies that the underwater waterfall falls over. Once you have seen it from the air, the rest of the coast arranges itself around it.
It is also the boundary between two completely different pieces of sea. Inside, water you could stand up in for hundreds of metres. Outside, ocean going down further than the island stands up. The transition happens across the width of the surf.
Four bands
Pale over shallow sand, turquoise over a few metres, olive over coral flats, navy past the reef. Those four bands repeat around every bank and channel on the coast.
How it changes through the day
Colour is a function of light as much as of depth, so the same lagoon looks quite different at nine in the morning and at four in the afternoon.
Low sun skims. A large proportion of the light reflects off the surface rather than entering the water, so the sea reads as glare and silver and the bottom disappears. As the sun climbs, more light penetrates, the sand under the shallows lights up, and the colour bands separate out. High sun is what the lagoon wants.
Wind matters too, and in the opposite way to what people expect. A perfectly flat surface is a mirror and can throw a lot of glare back; a light ripple actually scatters that reflection and lets more light through. What ruins it is proper chop, which breaks up the surface enough to blur everything under it. That is one of the reasons an operator watches the wind, and it is why the reliable slot is a calm morning rather than a still one.
What ruins it
Cloud is the flat killer. Under an overcast lid the light arriving at the water is diffuse and weak, the sand stops returning colour, and the whole lagoon goes grey-green. That is a fine day for a beach and a poor one for a scenic flight, which is exactly why a flight can be moved on a day that looks pleasant from the shore.
Proper chop is the other one. A light ripple is harmless and even helpful, but once the surface is broken up the light entering the water scatters in every direction and everything below it blurs. On a windward coast that is the variable the operator is really watching.
Both are worth understanding before you book, because they explain a decision that otherwise feels arbitrary. Being told your flight is moved on a warm bright morning makes no sense until you know that the thing being protected is not your safety but your view.
