At more than three miles long, it is the UK’s second longest bridge and crossed by 40,000 cars every day. It was built at a cost of well over £300million across arguably the most perilous body of water in Britain, the Severn Estuary, which has the world’s second largest tidal range.

With a tidal range of 50 feet, swift currents and fierce winds opposing the tides, it can create deadly conditions. And these very conditions have moulded what lies beneath when you cross the Prince of Wales Bridge, an extraordinary engineering marvel that connects Wales and England via the M4.

Unbeknownst to many, underneath lie a dilapidated medieval chapel and the wreckage of two barges, obliterated by a collision and an explosion that killed several men and set the Severn waters alight, reports Wales Online.

These intriguing features were explored in Britain at Low Tide on Channel 4.

The ‘chapel’ in ruins

It’s difficult to believe anything could have been built on land where the tide ebbs and flows today. But the ruins of a chapel can be seen, right in the heart of the water. St Twrogg’s Island is only accessible at very low tide. On top of the island is a ruined building, the purpose of which isn’t known for certain.

On Britain at Low Tide, archaeologist Oliver Hutchinson of Citizan said it could have been placed at the site to warn people off the rocks at the dangerous point where the Wye and Severn met. It could also have been a point where people tried to cross the river.

Charlotte Mecklenburgh of the Museum of London Archaeology spotted lots of “different phases” to the building, with one phase “clearly” medieval.

Researchers also learned the island is also known as “Chapel Island”. Professor Andrew Reynolds of University College London said the style of the building suggested it dates from the 13th century in an early Gothic style. Some of the rocks used in its construction showed tool marks.

Professor Madeleine Grey, of the University of South Wales, said the island may have been an island hermitage.

She said: “If you were a hermit in the early Christian tradition, you went out into the desert. You’ve a job to find a desert in Wales. So they had this idea of a desert in the ocean. It first crops up in 1290.”

Prof Grey also shared insights from a historical document on a programme, revealing that a bishop once granted a Benedictine monk permission to conduct services on the island, which became a pilgrimage hotspot in the 14th century. However, its decline in the 15th century may have been due to rising sea levels, according to Prof Grey.

The island could have continued to serve hermits until the Reformation and acted as both a hermitage and a beacon for navigators.

The wrecks of two barges

In a tragic maritime accident in 1960, two barges crashed into the Severn Railway Bridge, leading to its partial collapse. The bridge has since vanished entirely.

However, the two barges, named the Arkendale and the Wastdale, remain stranded together in the sand for over half a century.

The incident became known as the Severn Bridge Disaster. Five men were killed. The tankers were carrying black oil and petroleum spirit and the Arkendale was heading from Swansea to Worcester.

Although there had been no sign of fog at the start of the journey, it descended very quickly at around 10pm and the two barges collided with the bridge.

The cargo caught fire on the water and, according to this report of the disaster, the “river was ablaze for two miles”, leaving the men who were faced with jumping into the water with the fear of burning as well as drowning.

Chris Witts watched it all from the boat he was working on at the age of 16.

He told the programme: “Suddenly the fog rolled in. The Wastdale, he lost his bearings, and ended up alongside the Arkendale and they became locked together and came out into the fast-flowing tide and were taken sideways up the river towards the bridge.”

He saw a huge explosion, caused when two girders fell from the bridge on to the barges. The damage caused by the falling girders is still visible in the ruins.

The Roman crossing point

Today, the Prince of Wales Bridge and its older neighbour, the Severn Bridge, carry two motorways over the estuary, and the Severn Tunnel transports trains on the Great Western line underneath the water from Bristol and London west to Swansea and beyond.

But the oldest known crossing point is located from Beachley, near Chepstow and just beyond the Wales-England border, to Aust on the opposite bank of the Severn. This route was utilised by Romans to transport soldiers into south Wales, and there are records of it dating back to the Middle Ages.

In the 20th century, a ferry service used to ply across the river, ceasing operations in 1966. Remarkably, these ferries could accommodate about 16 automobiles albeit parked so snugly that doors remained shut throughout the journey across one of the most treacherous waterways in Britain.

The vanished village

The ruined chapel on St Twrogg’s Island is not the only one in the area. Nearby, in the village of Sudbrook, near Caldicot, is another ruined chapel, said to date from the 12th century, which would have been at the centre of a “very large village”. It’s likely the village would have been prosperous.

The village in that form no longer exists but in the 1930s, excavations nearby uncovered the remains of another medieval building. It’s possible the village succumbed to erosion. Records show it was thriving in the 15th century and the church was still being used in 1674 because there’s a record of a wedding being held there. But by 1720, it was no longer being used – a period of time which coincides with a series of severe winter storms.

But Sudbrook’s fortunes were revived with the construction of the current Severn Tunnel in the Victorian era, when several of the houses seen today were built by the engineer behind the project. The tunnel opened to traffic in 1886, when it was the longest railway tunnel in the world.

The abandoned underwater tunnel

In 1810, an attempt was made to create the world’s first underwater crossing, further up the Severn between Newnham and Arlingham near the Forest of Dean in Gloucestershire. The aim had been to transport coal and workers managed to get halfway across before the project was abandoned. But now, only some stone work remains.

The Queen opened the first Severn Bridge on September 8, 1966, when it was hailed it as the dawn of a new economic era for south Wales. F rom 1966 to 1996, the bridge served as the M4, until the Second Severn Crossing was opened

The bridge today

The Second Severn Crossing was opened on 5 June 1996 by Prince Charles and now carries the M4 motorway between Wales and England. It became the more prominent of the two bridges crossing the Severn Estuary. The original Severn bridge, opened three decades earlier, is now the far less busy of the two, carrying the M48 motorway and often having to be closed in high winds.

In 2018 the Second Severn Crossing was renamed the Prince of Wales Bridge in honour of the then Prince Charles. It has three  motorway lanes per direction and is less vulnerable to disruption in high winds.

The whole structure, including both approach viaducts, is more than three miles long.