Why is the Olympic diving pool green? All of the reasons for the mysterious, verdant pool

Everyone knows it's green; nobody knows why

The Olympic pool has gone green – by now, that’s well known. But what’s more mysterious is why it has happened.

Experts from across the world have been confused by the strange behaviour of the diving pool over the last couple of days, as it has turned from a sparkly, healthy blue to a verdant, pond-like green.

And still – two days after it first went that way – nobody is really clear why it happened. Here is our collection of the most dependable theories, as well as why they might be wrong.

Ink

This was a late entry into the competing explanations for what went wrong – though it was odd that nobody had thought of it before.

Fresh from winning a historic gold for Team GB, divers Jack Laugher and Chris Mears suggested that the pool had gone green because of the large banners that are placed in the side of the pool.

It’s an explanation that has some problems, like the fact that the green ink probably wouldn’t be enough to turn the pool such a strong colour of green. But it also makes an awful lot of sense: those banners are there, they’ve gone blue under the water, and the colours of the Rio branding and the pool itself are very similar.

This would be one of the most reassuring reasons, since it would pose no danger to the competitors and be relatively easy to fix, just by taking the banner out of the water and getting it clean.

But the fact that hasn’t happened yet would seem to indicate that officials aren’t convinced – or just haven’t thought of it yet.

Metals

Water, when it interacts with some metals like copper, has a tendency to take on a green hue. The various parts of the systems that keep pools regulated are made of metal, and the pool itself is obviously made of water.

If something has gone wrong, then that could easily have led to the discolouration of the water.

Urine (or anything else)

This was perhaps the most obvious explanation that many people leapt to: something else had come from the people getting into the pool, and discoloured it. Since the pool was blue before, it would make sense if that stuff was yellow – so some suggested that it was urine, and others said that it might be fake tan.

But the trouble is that the pool is just too large for urine, fake tan or really any other substance to make much of a difference to its colour.

Algae

This is still the most likely explanation. Algae – plants that grow in water – are usually kept at bay by chemicals and treatment of the water, but something appears to have gone wrong there.

That’s likely for a number of reasons: it’s the reason that things like ponds are usually so green, and the colour is mostly the same; the greening seemed to happen more when people were diving into the pool, and once it began it grew quickly; it would probably take something that was growing organically to fill the pool so effectively; and officials have said that the fact that the pool is outdoor and the weather has been hot have caused issues for those maintaining it.

But one problem remains: when officials said the problem was algae, they also said that it would be fixed within the day. It still isn’t – which implies that those officials were wrong, either because pool technicians hadn’t been consulted or made the wrong projection, or because it isn’t algae at all.