The 5th and final location of a recent day of solo exploring was this Grade II listed mid-19th Century lime kiln, hidden in woodland in the Norfolk countryside. Lined with brick and flint, and with two entrances and four chutes leading into the brick-lined pot. You can read a bit of the history of lime kilns and quicklime production in Great Britain at this wiki page.
One of the entrances to the kiln.
Inside the underground brick-vaulted chamber.
What remains of the burning chamber, into which layers of crushed limestone and wood or coal were built up onto the grate “eye” of the kiln below.