Seithennin had the most important job in Cantre'r Gwaelod - overseeing the sluicesSliding gates for controlling the flow of water. that protected the kingdom from the sea. But one night he got drunk and left the gates open. At high tide, the kingdom disappeared for ever.

According to Welsh legend, Cantre'r Gwaelod was a splendid kingdom whose land is now under Cardigan Bay. Its story is told in the Black Book of Carmarthen, the earliest written collection¹ of Welsh poetry. Giraldus CambrensisA medieval historian. "Cambrensis" is Latin for "Welsh". wrote about a drowned forest there.

The story is usually dismissed as a myth like that of AtlantisAn island or city said to have disappeared beneath the see. The legend goes back to Ancient Greek times. . But an expert on geography, Simon Haslett, and an expert on Celtic literature, David Willis, believe it is rooted in fact.

They argue that Cantre'r Gwaelod is one of two islands that used to exist off the coast of Wales. They are shown on the earliest surviving map of the British Isles, the Gough MapThe earliest surviving complete map of the British Isles. It is drawn on two pieces of animal hide which together measure approximately two by four feet..   

According to Simon Haslett, "The two islands are clearly marked and may corroborateSupport a statement or theory. It comes from a Latin verb meaning to strengthen. contemporary accounts of a lost land."

According to PtolemyAn early mapmaker. He lived in Egypt in the 2nd Century AD, when it was a Roman province., the early cartographerMapmaker., the Welsh coastline may have been eight miles further west than it is now. Haslett thinks people might have been forced inland by erosion or a tsunami.

Archaeologists have made extraordinary discoveries about lost civilisations. In 1986 they uncovered pits full of treasure from the 12th  Century BC in the Sichuan province of China. They had belonged to the Sanxingdui people.

Between 6,000 BC and 3,000 BC, the Trypillia people dominated what is now Moldova and created beautiful pottery. But every 60 to 80 years they would burn their settlements to the ground and start again.

One of the most spectacular discoveries in the Middle East was Petra, the capital of the Nabatean people. It includes beautiful buildings carved out of the rockface.

In Orkney, the skilfully built Neolithic settlement of Skara Brae lay buried under the sand until a storm suddenly revealed it in 1850.

Today some experts worry that our own society could collapse and disappear in a similar way.

Yes: The world is in a state of chaos, brought about by war, the pandemic, economic recession, feeble governments and dishonest politicians. Our civilisation is no better than those that came before, and could easily be forgotten.

No: The human race is making enormous advances all the time, and AI will make them even more spectacular. The society of Cantre'r Gwaelod would look pitiful by comparison. 

Or... Civilisation is about how people think and behave, not the technology available to them. People from past centuries would think we have ridiculously easy lives and lack their resourcefulness.

 ¹It is thought to date from the mid-13th Century.

Sluices - Sliding gates for controlling the flow of water.

Giraldus Cambrensis - A medieval historian. "Cambrensis" is Latin for "Welsh".

Atlantis - An island or city said to have disappeared beneath the see. The legend goes back to Ancient Greek times.

Gough Map - The earliest surviving complete map of the British Isles. It is drawn on two pieces of animal hide which together measure approximately two by four feet.

Corroborate - Support a statement or theory. It comes from a Latin verb meaning to strengthen.

Ptolemy - An early mapmaker. He lived in Egypt in the 2nd Century AD, when it was a Roman province.

Cartographer - Mapmaker.