Grotto, Underground Garden and Bridge about 60m to west of Dewstow House

The Grotto, Underground Garden and Bridge, located about 60 meters west of Dewstow House, is a significant 19th-century structure. The main drive features a balustraded segmental arched bridge that spans a winding stream, which flows from a stone Gothic arch. This arch leads into a small gorge, flanked by tiered rockwork on both sides, with the stream meandering through small channels on a level artificial rockwork floor, creating stepping stones beneath the arch. On the east side, there is a small semi-circular pool. An arched entrance provides access to a winding rockwork passage that leads into the underground garden.

Upon entering the garden, on the right side, there is a caved-in passage that opens into the underground garden to the east. The majority of the floor is occupied by a naturalistic pool, crossed by stone steps. The space features pillars of rockwork with niches designed for ferns, and at the northern end, there is a waterfall that is said to still function. The chamber is illuminated from above, although the glazing has been lost, leaving only the iron and reinforced concrete framework. The water supply for the gardens was meticulously controlled, with various standpipes and water cocks concealed within the niches. This garden was specifically designed for the cultivation of ferns, a specialty of Henry Oakley.