In the stillness of a burning hot day in June, a group of archaeologists crowds on to a motor boat to examine the shore of eight rocky islets.

These are Vietnam's Cham Islands, renowned for tropical beaches, granite cliffs and the swallows now circling dizzily overhead.

Their names reflect their shape and character - one is called “East Wind”, another “Tomb”. That is the first clue to the history of these waters.

These are ship-wrecking seas, which is why the archaeologists have travelled to the tiny archipelago from the US, Australia and Japan.

They are here as wreck detectives.

Last year they arrived too late. The monsoon lashed the coast, and they could barely see across the narrow channel from the port of Hoi An, on the mainland, let alone sail across it.

But they were able to talk to marine rangers who patrol the surrounding Unesco biosphere reserve, and heard tales of strange objects lodged in the sea bed.

This year they have come earlier, before the monsoon, armed with permission to investigate.

As the boat enters a bay, the leader of the group stands alert, eyes squinting in the bright sunlight.

“Stop here. This place is interesting,” he says.

Australian Mark Staniforth would not have looked out of place at the helm of one of the schooners that careered through this channel 400 years ago.

In four decades as an underwater archaeologist he has learned how to read the coastline. For centuries, he says, ships must have moored in this bay for shelter from the wind.

“On a day like this, that’s fine,” he drawls.

 But then you get a typhoon.”

He paints a picture of vessels suddenly vulnerable as they get pounded by waves. If a ship breaks anchor or if it cannot sail out of the bay it will end up dashed on the rocks or beached.

“At some stage in history, that happened. Probably several times,” he says.

Another clue is provided by a modern shipwreck - a rusting hulk embedded in the sand.

Nobody expects to chance on a wreck, but the members of the team slip into their diving suits, and then, like a pod of elegant deep-sea creatures, they disappear, leaving only bubbles popping on the surface.

All trained under Staniforth in the colder, darker waters off southern Australia and they have imbibed his respect for the tombs of the deep - every broken bit of ceramic, every plank of timber is important.