As part of the extensive renovation of a 19th-century mansion, a new subterranean spa provides a place for exercise and relaxation. The network of underground rooms is intended to be a strong contrast to the decorative character of the family house above and discovered via a secret staircase accessible from the ground floor. Defined by the simple sculptural treatment of stone, plaster and water, a subtle mixture of natural and artificial light animates the space and creates an immersive and sensual environment. The main 17 metre long swimming pool, lined with coarse textured black basaltina, offers a strong visual and textural contrast to the smooth surfaces of the white limestone-lined blocks. The distinctive stone finishes of each space create a labyrinth of treatment rooms- sauna, spa and gymnasium. The undulating form of the plaster ceiling hovers above the pool and is modeled by a halo of indirect light.