"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.
This picture is of the second reserve, to the southwest of the first one. You can see the four concrete bases which would be used to erect a single tower when it became necessary actually to use the reserve, the access hatches at right, and two air vents. Probably till then the area 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 receiver.
There are some more pictures of the buried reserves here but I am not sure exactly what they are of.
Peter Facey, Winchester, England
20151017 originated