The sludge forms when sea algae is overloaded with nutrients by warmer temperatures and polluted water.
HARRIET HADFIELD: At first glance, it looks a bit like the tide's out, revealing a muddy shoreline But this is not silt. And it's not meant to be here. Marine mucilage, also known as sea snot, has blanketed this Turkish coastline now for days. Seen here under the water clinging to rocks and seaweed, you can understand the problem it's causing to sea life, it's alarming both for fishermen and for local people.
MUSTAFA YUCEL: The root cause of this, the sea snot, is really we need to limit the amount of anthropogenic-derived [? agents ?] that are entering the sea. We need to stop or slow down climate change, the warming, because one way or another, it's going to affect our seas.
HARRIET HADFIELD: The slimy layer is formed when algae is overloaded with nutrients. It's a natural process but one that has got completely out of control. Campaigners have blamed global warming and have called on President Erdogan's government to step in.
RECEP TAYYIP ERDOAN: Just like we cleaned Golden Horn, which was a greater trouble, hopefully we will save our seas from the trouble of mucilage. My fear is if this expands to the Black Sea and comes to Mamure, from there, the trouble will be enormous. We need to take this step without delay.
HARRIET HADFIELD: Work has begun to try and remove the sludge. But this is a massive problem for the city to tackle and one so far for which there doesn't seem to be an obvious solution. Harriet Hatfield, Sky News.