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"Buried Reserves" were backup systems to be used in the event that the main system was rendered inoperable by enemy action. There would be one buried reserve to transmit, and one to receive. At Rye, they were both still more or less intact in 1983, and seem to be so today in 2015. I have no idea what, other than water, is inside.
You can see three air vents, an access hatch at right, and beyond four concrete bases which would be used to erect a single tower when it became necessary actually to use the reserve. Probably till then it would have been covered with camouflage netting; the reserve is quite obvious today on aerial photos.
I am unable to deduce from what remains whether this reserve was to transmit or receive. However, if the reserves followed the same placement as the main towers, then this one would have been the transmitter.
There are some more pictures of the buried reserves here but I am not sure exactly what they are of.
In use the buried reserve would have looked like this:-
Peter Facey, Winchester, England
20151017 originated