Painshill Park, Surrey
The Grotto had fallen into a poor state by the mid-20th century. A huge section of the roof collapsed in 1948, and the main chamber was filled with debris.
The Grotto, first mentioned in 1770, was built for Charles Hamilton by Joseph Lane of Tisbury in Wiltshire. It is constructed across two islands in the lake and consisted of a brick shell covered with Cotswold limestone.
A number of contemporary illustrations and sketches, by Frederick Magnus Piper, Elias Martin and William Gilpin, show what the Grotto looked like, and a 1937 photograph shows the detail of the crystal ceiling.
Inside the main chamber, hundreds of man-made glittering stalactites hung from the ceiling. Its walls and ceiling were decorated with crystals, shells, gypsum, coral and other coloured minerals, and the floor was of sand and crushed shell.
Archaeological investigation and recording work
Excavations, which began in 1982, revealed mauve fluorite and orange calcite crystals from the original ceiling, cockleshells from the floor and some false stalactites surviving beneath the rubble. Some remains of the slate roof were found, but the lead flashings and gutters were missing, suggesting that theft of these items may have caused the roof to collapse.
Careful recording of the floor during excavation revealed which areas of floor had been worn down more and helped to identify the 18th century route that visitors had taken around the Grotto.
The archaeology team also revealed the feature's water system, including the pools and cascades within the Grotto. Beneath the floor, they discovered the network of pipes that had channelled water from the lake via a small pump room and then through the Grotto's cascades and rock pools before returning it back into the lake.
The task of restoring the Grotto began in 1986. In Hamilton's time, the Grotto islands were still linked to the mainland, until the flooding of the lower half of the lake, which would have made construction much easier than restoration is now.
In 1992 the roof of the Grotto was replaced.
Reconstruction of the ceiling is still in progress, with a frame of green oak holding new stalactites of lath and plaster.
The lath and plaster surface is coated with lime mortar and the many thousands of crystals are being painstakingly applied by hand.
The long and complex task of restoring the Grotto still continues (2008).