For centuries it has lain undisturbed beneath the waves, just a stone's throw from one of Britain's best-loved beaches.
Stretching more than 280 yards along the sea bed, this bizarre V-shaped structure is a giant fish trap, used at the time of the Norman Conquest to catch hundreds of fish without the need for a boat, rod or net.
The ancient structure - discovered by archaeologists studying aerial photographs of the West Wales coast - is so large it can now be seen on Google Earth.
Scientists believe it is one of the biggest structurse of its kind.
The fish trap is submerged at low tide and no longer catches fish. But in its day, it was designed to act as a natural rock pool - trapping fish behind its rock walls as the tide flowed out.
Dr Ziggy Otto, a diver and lecturer in the coastal environment at Pembrokeshire College, believes the trap is around 1,000 years old.
'It is an amazing structure,' he said. 'It looks well defined on the photographs, but when you are in the water it looks just like a natural reef.
'There can be little doubt that this rather impressive, and quite apparently man made, structure is an ancient fish trap. The structure is entirely underwater at all stages of the tide.'
The trap is just 12ft deep close to Poppit Sands on the Teifi Estuary in Dyfed. Dr Otto believes the walls are made of locally quarried rock or boulders brought down to the coast by glaciers during the last ice age.
The trap's walls are covered in algae, worms and sea anemones. The wall is around three feet wide, and only the top foot is exposed. The researchers are unsure how tall the original trap was - and how much is buried under the shifting sands.
The V-shaped structure has a gap at its point where fisherman would have placed nets to catch fish. They could also have blocked up the gap, and then scooped up fish trapped in the shallows.
The trap could have been used to catch migratory salmon and trout as they swam up the Teifi, said Dr Otto.
Fish traps, or fish weirs, were common and controversial in Britain 1,000 years ago. They were so effective at removing fish from rivers, the Magna Carta banned them - allowing them only on the coast.
Louise Austin, of the Dyfed Archaeological Trust, said: 'Fish traps were a widely used means of catching fish in the past which made a significant contribution to the economy of many coastal and estuarine communities. Today only a few are known to survive in Wales.'
On an exploratory dive, archaeologists discovered that the trap was acting like a natural reef and was covered in tube dwelling worms, red algae and sea anemones.
They plan to visit the trap again tomorrow.
Although it was recently spotted on aerial photographs, an armchair archaeologists could have discovered the trap for themselves on Google Earth.
Google said the V-shaped structure has been visible on its collection of satellite and aerial photos since at least December 2006.
'It's true that many amazing discoveries have been made in Google Earth - a pristine forest in Mozambique that is home to previously unknown species, a fringing coral reef off the coast of Australia, and the remains of an Ancient Roman villa, to name just a few,' a spokesman said.
'Everyday we're impressed and inspired by what eagle-eyed armchair explorers and scientists alike discover about our world using this technology.'
Last month, a mysterious grid of lines in the Atlantic was spotted on Google Earth 600 miles off the coast of Africa. The discovery set internet bloggers buzzing with excitement and appeared to baffle Atlantis experts.
However, the grids turned out to have been created when the Google map was created - and do not exist in the real world.
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