The waters in the area of the Omega Bay near the city of Sevastopol in south-western Crimean Peninsula changed its colour to the vivid green hue for reasons unknown.
Some reports that circulating locally said that the water changed colour after an unknown man threw a plastic bag containing some unidentified liquid into the water.
The bright green stain quickly spread in size and the colour but it is not understood why it remains so clearly defined on the sea bed.
Local officials have confirmed that it is not the first time water has changed its colour to this psychedelic green in the past few weeks.
A Russian health watchdog is investigating but has so far reached no conclusions.
Some online commenters speculated about the cause of the colour change.
Netizen ‘Masha’ commented on the video of the bright green sea patch: “I had an aquarium when I was younger, and usually water changes its colour when you add a special drug for fish.
“Based on the amount of dead birds this year, maybe somebody has decided to heal the fish.” ‘trjgt45’ added: “I hope they will find out what is really going on soon, that is pretty spooky if you ask me.”