From our research in the national archives we found the below. It is an extract from an US Army interview with Oberst Rudolf Graf von Schmettow, commander of the Channel Islands (27 September 1940-20 October 1941 & 1 September 1943-28 February 1945) in which he mentions the Tunnels.
"The geological structure of the islands, which are known to consist of solid rocks reaching 140 meters undersea level, made it possible to build cave-like underground systems on a large scale almost everywhere on the islands.
Thus, especially in the interior parts of the islands, very strong , reinforced underground installations, which were distributed according to tactical view points, could be built for housing or assembling reserves. They offered very strong protection from ship based artillery fire and aerial bombs.
They were frequently lying under 20-30 meters of natural rocks, situated in deep valleys they could not be recognised from the air.
The entrances were prepared for defence.
The spoil obtained by mining made it possible to transport less construction material by sea, needed on very large quantities for fortress construction, road building (approach roads).
Most of the caves were built with a large profile, they had mostly 2-3 entrances permitting close columns eg for ammunition supply, to enter and leave.
Their inside was improved with concrete, light, water pipes and ventilators were laid as well as drainage systems, where ever necessary so that dry housing and storing of ammunition and supplies was possible.
The value of underground installations was first recognised on Jersey, there they were began first and perfected to the greatest extent.
The planned system could not fully be accomplished, especially on Guernsey and Alderney, for various reason, such as the lack of special apparatus and equipment as well as fuel, and also because the rock construction and mining companies had to leave in 1943. Netherltheless it was possible to complete on Guernsey and Jersey the modern hospitals and to make use of them, and also to store most of the ammunition and other supplies as well as to make it possible for the reserves to assemble there."