During works on modern maritime defences in Hampshire, part of a Victorian promenade was unearthed.
Under the asphalt near Southsea Castle, workers found a 2 m thick (6 ft) stone surface.
The old walkway was built in 1848 and is believed to be the original edge of the château ‘s security tunnels.
It has been “an important part of the local narrative,” said Wessex Archeology.
Boring holes on the existing footpath to explore the depth of the underlying tunnels was carried out by contractors in early studies for a £131 million project to construct new sea defences.
They had hoped to fine gravel but found solid stone instead.
Royal Haskoning DHV’s Richard Samphire reported that the result was a “great surprise.”
“It’s a big step in our perception of the history of South Sea, as we didn’t know until now what was between the footpath and the tunnel roof around the castle,” he says.
The tunnels underneath the promenade were part of a redesign of Castle Southsea, at the beginning of the 19th Century.
The road outside the castle used to be an active military base since the middle of the century.
Wessex Archaeology consultant Naomi Brennan said the old walkway was thought to have been lost when new surface was added.
“The discovery illustrates the transition from a mere military site into a trendy resort of the 19th century in South Sea,” she said. “It is a significant part of the local storey.
The sea conservation scheme includes the development of barriers, the raising of land and the extension of beaches along 4.5 km of coastline.