Drone footage has captured the "tragic" moment a pod of dolphins appeared to swim into an oil slick off the coast of San Diego, California.
Shared to Instagram on Saturday (local time) by whale watching captain Domenic Biagini, the aerial video shows at least eight animals attempting to navigate the seemingly polluted waters.
Rainbow colours from the oil slick shimmering across the surface of the ocean are broken as members of the pod breach the surface.
“This is the most tragic thing I’ve ever filmed,” Mr Biagini wrote.
The “shocking discovery” was made as Mr Biagini headed towards San Clemente Island where blue whales were observed the previous day.
Instead of finding “feeding giants”, he said the area was deserted.
Investigating further, Mr Biagini said he travelled south and that’s when he came across the oil slick.
“Tragic doesn’t begin to cover it,” Mr Biagini wrote.
“For more than 50 miles (80 kilometres) the ocean was covered in a thick sheen of oil, a devastating tragedy for the environment."
He added: "I truly hope this isn’t as bad as it appears, but if this oil is already in an area that spans at a minimum of 70 miles (112 kilometres), we could have a catastrophe of unimaginable scale on our hands."
Authorities aware of spill off San Diego coast
National Oceanic and Atmospheric Administration (NOAA) confirmed to Yahoo News Australia they were aware of the incident, but had not been asked by the US Coast Guard for scientific support.
The US Coast Guard are aware of a separate spill of at least 378 litres nearby which they expect to dissipate, according to the San Diego Tribune.
In sharing the video, Mr Biagini said very little was known about the incident, but he hopes his post went viral so that answers could be found.
The post has attracted more than 19,000 likes and close to 1000 comments, with many describing its content as "heartbreaking".
"Humans are a virus in this beautiful planet," one person wrote.
"This is so hectic. Need answers ASAP," someone else said.
"Why is there no respect for life underwater? Definitely need answers," another person added.
The US Coast Guard and the Environmental Protection Agency have been contacted for comment.
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