As part of recent ground works for the sea defences near Southsea Castle workers discovered a stone surface two metres thick under the tarmac.
Further investigation revealed this to be the original front edge of the old promenade, which was constructed in 1848.
Richard Samphier, associate director of contractors Water Europe from Royal Haskoning DHV, said: 'This discovery was a huge surprise as we were expecting to find gravelly material below the tarmac - not two metres of solid stone.
'It's a huge leap forward in our understanding of Southsea's history as, until now, we didn't know what was between the footpath around the castle and the roof of the tunnel.
'There are very few historical records relating to the construction of the tunnel or adjustments to the promenade, so we now need to undertake more archaeological research to fill in the missing pieces of the jigsaw.'
Workers were initially boring holes in the tarmac footpath to investigate the depth to the tunnels below. The tunnels beneath the promenade form part of a redesign of Southsea Castle undertaken in the early 19th century that allowed defenders to fire into the moat at attackers from all sides.
Radar was used to ensure there was no damage or interference with the discovery.