A vast “raft” of volcanic rocks stretching over 150 sq km (93 sq miles) is drifting through the Pacific Ocean, scientists say.
The sea of pumice – the size of 20,000 football fields – was first reported by Australian sailors earlier this month.
Experts say the mass likely came from an underwater volcano near Tonga which erupted around 7 August according to satellite images.
Sailors have been warned to stay clear of the potential hazard.
Pumice is a lightweight, bubble-rich rock that can float in water. It is produced when magma is cooled rapidly.
Large “rafts” of the volcanic rock are more likely to form when a volcano is located in more shallow waters, say experts.
An Australian couple sailing their catamaran to Fiji were the first to report the “pumice raft”, after inadvertently entering the rubble at night.
“The waves were knocked back to almost calm and the boat was slowed to one knot,” Michael Hoult and Larissa Brill wrote online on 16 August.
“The rubble slick went as far as we could see in the moonlight and with our spotlight.”(independen)