"We are also not allowed to offer the athletes any tactical advice, " says mopper Samuel Bevan, 18, from Carmarthen in Wales.

WSJ: When These Olympians Say Their Job Is 'No Sweat,' They Mean It

The result, marketed under the name of Bauxsol by Virotec International, a former gold-mining company, has proved to be an effective mopper-upper of mining waste.

ECONOMIST: Mine pollution

"We try to stay out of the way it would be a disaster if we got caught in a game, " said mopper Chris Stainton, 22, from Manchester.

WSJ: When These Olympians Say Their Job Is 'No Sweat,' They Mean It