Grotto and retaining walls. Mid C18. Rough hewn rocks, some brick. Circular plan. Central blocked archway, flanked by single walls. On the lake side is a further archway with pendant keystone. Interior, which has some red brick, is domed and has 3 large and single small arched niches. Each with rough hewn keystone and voussoirs. The domed roof was originally lined with mineral crystals and hung with stalactites. Flanking the entrance are retaining walls, extending 15 metres to the right/south and to the north/left, 31 metres running between the lake and wooded bank. (NCC/js)