In this picture taken with a fish-eye lens, the water of the diving pool appears a murky green in the Maria Lenk Aquatic Center. (Matt Dunham/AP)

It’s primordial, this pool. You expect a cold, dead hand to come jutting up out of it. The Olympic diving tank is dark and seemingly bottomless, and as a rule in life, nothing good grows or happens in dark-green water. Things that are green and wet: spoiled kale. Decomposition. Industrial subatomic runoff. Old ceiling leaks. Dead seaweedy things that clutch at your ankle.

Would you wade in that, much less put your face in it? Mothers all over the world are worried about that pool: They know what the toddler section of a public aquatic park looks like late in the day. Anyone who has stared at the bottom of an old fountain is worried about that pool.

“The swamp pool,” British gold medalist Chris Mears called it.

The “green lake,” German diver Patrick Hausding labeled it on Twitter, along with a photograph of him holding his nose.

The outdoor tank at Maria Lenk Aquatics Centre was so sickly colored and dank Friday morning that organizers closed it and canceled practice while they tried to treat it — a first in Olympic memory. “Hope we haven’t been diving in anything too bad the last couple of days!” British diver Tom Daley tweeted.

Yet the organizers insisted it was perfectly safe. It was nothing, just a small litmus issue. “We learned chemistry is not an exact science,” Rio 2016 spokesman Mario Andrada said.

Indeed! Chemistry is so casual. So easily fixed. We’ll just change the makeup of H2O. Make it three parts hydrogen and one part oxygen.

Leave it to these folks, and you get the feeling the next time someone sticks an arm in the pool, they will never get it back. The flesh will dissolve.

It’s only partly funny, of course, because the athletes here have to submerge their eyes, ears and noses in that stuff. In reality, it’s a scandal — and a total embarrassment for the International Olympic Committee, which so determinedly ignored Rio’s obvious financial problems and the organizers’ chaotic struggle to build its venues on deadline. Their insistence that the water has tested chemically safe would sound more reliable if they hadn’t issued so many shifting explanations. And if so many of the waters around Rio hadn’t been infected with viruses and bacteria from urban runoff and if open sewers didn’t surround the Olympic Park, competitors and spectators catching whiffs with every breeze.

On Monday the diving pool was a normal crystal blue, but something happened overnight — something Stephen King-ish. The next morning it was the color of a spilled isotope.

At first, organizers attributed it to “a proliferation of algae caused by the heat and lack of wind in the venue.”

University of Virginia physics professor Lou Bloomfield explains how divers use gravity and change their shape to execute the perfect dive. (Thomas Johnson,Julio Negron/The Washington Post)

During the women’s synchronized 10-meter event, bronze medalists Meaghan Benfeito and Roseline Filion of Canada climbed the tower and looked down and almost started laughing.

“It was weird,” Benfeito said.

They told themselves, “It’s water. We know that it’s water down there.”

Benfeito said, “Don’t open your mouth. Just in case.”

A day later, the color had spread to the water polo pool on the adjacent open-air deck. Andrada suggested the problem was that perhaps too many people used the pool — a hazard impossible to anticipate at an Olympics — which caused a sudden “change in alkalinity, that was the reason.” He then added, “We expect the color to be back to blue shortly.”

By Thursday, U.S. men’s water polo team captain Tony Acevedo said, “I could barely open my eyes” in the water, and other competitors were complaining of itching and burning. Andrada claimed the water was actually “improved” and also joked that it was the same color as Brazil’s flag.

By Friday morning, the green murk was almost a solid, and the pool was closed. Athletes were told to perform “dry training.” Presumably, this meant they were to jump from the springboard onto concrete. A statement from organizers said nonsensically, “The reason is that the water must be still so the pool can return to its blue color as soon as possible.”

Whatever you do in a swimming pool, don’t move in it.

This time at the daily media briefing, Andrada said the pool had been over-treated with chlorine. Also maybe it was the weather that was to blame: “The rain doesn’t help,” he said.

Because, you know, rain is so green.

Throughout, the organizers have insisted the water is clean, no matter what the athletes may feel — or smell.

“We reiterate that the water doesn’t threaten the health of the athletes,” Andrada said. “One or two complained about their eyes being itchy. This was a result of using more chlorine, so we reduced the quantity. We retested the water, and it was within the [required] parameters. . . . Some things have taken longer than anticipated.”

All in all, it’s a menacing mystery, and the official explanations get only more meandering, while the athletes, who clearly don’t believe them, clamp their mouths firmly shut and bravely take the plunge. Some of them have devised their own hypotheses: Britain’s Jack Laugher told ITV he suspected the pool was infected by ink that had bled from the decorative plastic boards surrounding the pool deck.

Whatever the reason, by Friday afternoon the water was still a dismal green, and divers were back in it for the preliminaries of the women’s three-meter springboard. Still, they had it better than the next group of athletes scheduled to perform in it: the synchronized swimmers. The synchro athletes rely heavily on timing and spotting underwater, and they have to keep their eyes open for long periods, with no goggles. They like to see the bottom of the pool so they don’t bang their heads on it or each other.

But never fear. Organizers expect the pool to return to blue at any moment.