IT is known as sea sparkle, but some have wryly suggested it is the lights from a dolphin rave or shark party.
Others say it is simply the magic of the ocean, but onlookers could be forgiven for thinking the sea was on fire.
Scientists have a different explanation for why waves off Newcastle Beach were glowing luminous blue at night this week.
University of Newcastle marine science lecturer Troy Gaston provided a rational explanation for the natural light show.
"It's an algae that is common off our coast in summer," Dr Gaston said.
"It produces bioluminescence naturally."
Dr Gaston said algal blooms of Noctiluca scintillans occur "after a few days of north-easterly winds".
The wind moves nutrients to shore from deep water off the continental shelf, feeding the algae and causing it to bloom.
"High concentrations are needed to see the waves glowing blue," Dr Gaston said.
The algae was a form of phytoplankton that was basically "plants floating around".
The Newcastle Herald reported on Thursday that the algae looked like tomato soup in daylight.
But at night, the waves were electric-blue.
Several theories abound about why the species evolved to glow, with some saying it helped them survive.
"It may be to scare off predators," Dr Gaston said.
"In the deep sea where there is no light, they have to create their own light.
"They may just do it because they can."
Large blooms signalled the algae was close to the end of its life.
"It decays and disappears," Dr Gaston said.
The algae appeared along the coast from the Hunter to Batemans Bay, including Bondi and Coogee.
Newcastle Beach glowed blue on Wednesday night, but yesterday there was no sign of the algae, a lifeguard said.
NSW Office of Water spokesman James Muddle said the algae had no known toxic effects, but it may cause skin and eye irritation in some people.
Mr Muddle said the algae was always in the sea, but in varying concentrations.
"The last time we had a bloom of this significance on the eastern seaboard was 2010," he said.
Weather and ocean conditions were right this week for "a mass accumulation".
"It's at the whim of wave action, tide, currents and wind," he said.
"It might be on the beach for an hour then totally disappear."