Incredible photos of Rio de Janeiro's heavily polluted waters

The Olympics begin in mere days - and Rio de Janeiro's waterways are still teeming with trash, bacteria, viruses, and human sewage.

A 16-month study commissioned by the Associated Press has found that polluted water at many Olympic venues and tourist hotspots could cause "violent" illness. The highly infectious adenovirus, for example, was detected at 90% of test sites.

The findings prompted one expert to give tourists and athletes an ominous warning: "Don't put your head underwater," Dr. Valerie Harwood, the chair of the University of South Florida's department of integrative biology, told the AP. Athletes competing in water events are being told to "keep their mouths closed."

Here's what Rio's dirty waters look like up close, and from above.