Initially I thought this may have been related to the Kingsway / Queensway Tunnels. Many years ago I had the opportunity to visit the Queensway tunnel however through an odd route. All there was keeping the access hole covered was a thin loose metal plate, with steps below into the tunnel.
when yo go into the tunnel its built up at the portal if you look.about 100yrds in it drops down to the tunnel floor,in a slope.....about 13ft........water