A burst sewer? Divers relieving themselves during competition? Or just general foulness related to the standard of facilities at some of the Rio Olympics venues?
None of the above, say officials, in response to questions about why the diving pool at the Maria Lenk Aquatics Centre has turned a shade of green strikingly similar to the official colour of the Rio games.
Most worryingly for competitors, however, is that the water in the polo pool, just metres away, remains a healthy-looking shade of blue.
Officials have reassured competitors that there is no risk to their health, but at least one competitor said she could not see her partner underwater.
British diver Tonia Couch told reporters that the pool’s water was so green that she was unable to see her partner Lois Toulson underwater, The Times said on Twitter.
Reuters reported that Rio 2016 organisers said tests of the water in the diving pool found it posed no risk to health, although they were still investigating.
The problem may have come from a faulty filter or problematic water quality.
“To ensure a high quality field of play is mandatory to Rio 2016 committee,” an official statement said.
“Water tests at Maria Lenk Aquatics Centre diving pool were conducted and it was found to be no risk to athletes’ health. We are investigating what are the causes of the situation but are pleased to say the competition was successfully completed.”
US chemistry expert Dr Raychelle Burks says the explanation could be simple and anyone with a pool in the backyard may have confronted the same problem.
Pool & spa care websites have covered the #greenpoolmystery. Algae or oxidized copper is typically the thing. No alien goo or ectoplasm
— Raychelle Burks (@DrRubidium) August 9, 2016