Just past the Observatory on Bidston Hill
coming from the side that leads to the entrance to the Observatory, following the wall facing Bidston Windmill, you will see a path leading to a stepped area (down about four feet). Immediately on the wall left and right of this stepped area are the carvings. To the left (looking towards Seaforth), if you look carefully about twenty - sixty feet past where this stepped area is you will see a concreted area around which is sandstone. This is (or was) the entrance to the tunnels. You might need a pneumatic drill to break through to the tunnels beneath...!
There is another entrance down the hill on the Wooded area (facing Birkenhead) where there is a bricked-up natural looking opening into the tunnels Best of luck! linuxfanatik