Bikini-clad models, paddle-boarders and romantic couples are using a Novosibirsk energy plant's ash dump site - nicknamed the local "Maldives" - as background for their snaps, posing as if relaxing in a holiday paradise.
But the Siberian Generating Company, which owns the site, says the lake's bright colours are due to calcium oxides - substances found in quicklime and harmful to humans - diluted in shallow waters.
Although the company has hung warning signs, the photographers keep coming, driven by the quest for Instagram "likes".
"The rash is gone already, but I would not recommend tasting this water," said Alexei Cherenkov, whose photo on a unicorn float was "liked" almost 400 times.
"I went there for a pretty picture. Our city is grey and this is one of the available 'beautiful' places," he told AFP.
Cherenkov said there was no security, and the lake attracts many weekend visitors. "Some people even go there for barbecues."
Photographer Yekaterina Aksyutina went to the lake with clients - a couple who posed for romantic photos in wedding attire.
"It was an important date in their relationship and they asked for a photo session there," she said.
"Nobody went for a swim or touched the water, we know it might be dangerous."
The Siberian Generating Company has cautioned that the site is off limits.
"The territory is an industrial zone, not a nature reserve or aquatic park," it says on its website, warning people not to go into the water or use it to water plants.