When sailors Michael Hoult and Larissa Brill saw land in a remote corner of the South Pacific they had to double check the charts.
No, they weren't imagining things. Almost overnight a massive "floating island" the size of Manhattan appeared near Tonga, and is floating slowly southward.
The raft of volcanic pumice is measures almost 150 square km and was discovered by the Australian sailors earlier this month.
According to the BBC, expert reports form satellite monitoring services estimate the landmass emerged around August 7.
Produced by an enormous underwater volcano off the shore of Tonga, the eruption has been releasing volcanic pumice stones.
The porous stone, formed by rapidly cooling lava has tiny gas bubbles inside, causing it to be soft and buoyant enough to float on the surface of the ocean.
The sailors Hoult and Brill from Australia were the first to encounter the "raft" of pumice when relocating their catamaran to Fiji.
Sailing through the waters near VaVa'u they said it took "6-8 hours" to traverse the enormous pumice raft.
"It was like ploughing through a field," they wrote. In their videos the surface of the water looks like land.
They estimated the rock must have been half a foot deep in places, and potentially hazardous to other vessels.
Sharing a report to Facebook Michael showed a picture of a "basketball size" chunk he had fished out of the water, which could clearly do damage to other vessels. They gave the location of their sighting as 18 55' S, 175 21' W,near the Tongan island of Vava'u.
"The waves were knocked back to almost calm and the boat was slowed to one knot," they described the eerie effect of how the rocks calmed the water, as they slowed their craft.
Although their rudder was temporarily jammed, they managed to free themselves from the rubble.
The field is a long way away from New Zealand, but slowly drifting south. Experts have predicted the course of this floating island will move it towards Australia. It is expected to reach Queensland, Australia at some point in 2020.
Associate Professor Scott Bryan for the Queensland University of Technology, said that large pumice events were not uncommon in the area.
Talking to the BBC, the professor said "At the moment there are more than a trillion pieces of pumice all floating together, but over time it will break up and disperse across the area."
While this could be dangerous for pleasure craft in the area, the pumice field is overall a positive thing for marine life.
Those who have studied the phenomenon say that the floating islands are a rich habitat for life.
In particular, the coral fields of the Great Barrier Reef are set to benefit from the arrival of the floating rocks.
"This is a potential mechanism for restocking the Great Barrier Reef," Bryan said.
"Based on past pumice raft events we have studied over the last 20 years, it's going to bring new healthy corals and other reef-dwellers."
Life forms "can attach themselves to the pumice and be transported thousands of km away. So it's a way to renew ecosystems somewhere," Dr Jutzeler said, speaking to the BBC.