Bioluminescence plankton was seen in the water on Southend beach. The rare sight was seen at around 1am by two people out for a late night stroll.
The blue light is from tiny marine organisms that can sometimes cause the surface of the water to sparkle at night, when they are disturbed.
The sea glowing phenomenon is usually restricted to tropical destinations like Thailand or the Maldives, but it can on rare occasions be seen here in Britain.
Local resident Gavin Sager said: “I found some of these a couple of years ago here in Southend, it was absolutely beautiful!”
And Ben Hibberd added: “I’ll never forget being out on a boat in this! The boat cutting through the water and the hull glowing, it was amazing!”
However others are concerned that the plankton could be related to untreated sewage that has reportedly been discharged into the water recently. Mike Halladay said: “I thought it was because of all the sewage being pumped into the sea by Anglian Water from last week.”