so after many years of it being open then sealed then ripped open again i ventured back down again with better torch and camera and a bit more savvy on photography..well a bit!
"Behind a sealed entrance in Noah's Ark Rd, a set of steps lead in to an air raid tunnel which then slopes gently downwards towards Coombe Valley. The tunnel would have been able to accommodate approximately 850 people at a depth below ground level that varied from 30' near the entrances to around 100' under the highest point of the hillside. The Coombe Valley entrance to the tunnel was, when I last visited quite a few years ago, situated at the rear of R J Barwick & Co. and was kept securely locked. Mr. Richard Barwick kindly granted me access at the time - the first stretch of this end of the tunnel was well lit by overhead strip lighting and was used for storing various bits and pieces. I've no idea who occupies that site these days, maybe a local contributor knows more?
The tunnel was originally constructed prior to WWII as a heading for a water main, but with the onset of war the height and width of the heading was enlarged and the 8" water main covered by a concrete floor. Access to the main was kept via manhole covers at frequent intervals. Electricity was supplied, fresh water was available via connections to the main, and sewerage points were also constructed.
Lined with concrete throughout, the tunnel is 6' wide and 7' to the crown of the arched roof, and it falls from the Noah's Ark Rd end at a gradient of 1 in 20. Blast walls were constructed at intervals throughout. More statistics? The Borough of Dover Air Raid Precautions proposal for this site, dated 20th February 1939, give a total length of 379 yards, effective length 318 yards and the estimated construction costs were £7960. A further tunnel was suggested linking this one to Edred Rd which would have sheltered a further 1275 people for only a few pennies more (well, another £8560 actually) but it was never built.
During the 1950's the Coombe Valley end of the tunnel lost approximately 150' of its length when the embankment was cut away to make room for a new gas holder. The site was earmarked by the Home Office for re-use as a shelter in the event of war with the Soviet Union"
History borrowed from this source
sussex history forum
Few more recent pics of this place