A patch of red algae almost 500 miles long is floating off the coast of Brazil.
Visible from space, the slick stretches from Rio de Janeiro, past Sao Paolo to the southern state of Santa Catarina.
It has been caused by the hottest weather on record in some parts of the country, with decomposing algae eaten by micro-organisms called Myrionecta rubra, and then killed off by rising sea temperatures.
The algae can pose a serious threat to marine life, as it is toxic if ingested in sufficient quantity, and the large blooms can suck up oxygen, suffocating ecologies.
Oceanographer David Zee told Sky News that only a large storm could now break up and disperse the vast mass.
But with the water at the surface at 30C and the hot weather set to continue, the red slick is unlikely to move on soon.
Over recent days beaches near Rio, where people are heading to escape the heat, have been deluged with sticky foam.
Scientists at the University of Hull have conducted experiments using ultrasound blasts to break up such masses.