Architecture studio MAD has created five installations inside the Kiyotsu Gorge Tunnel, a mountain viewpoint in Japan's Niigata Prefecture, including a lake framed by mirrors and a foot bath in a wooden dome.

Beijing-based MAD was invited to transform the decades-old tunnel for the Echigo-Tsumari Triennale 2018, which creates both temporary and permanent artworks across 200 villages in the rural region.

The intervention consists of five different immersive artworks, each designed to represent an element found in nature: water, wood, earth, metal and fire.

Along with the lake and foot bath, the installations include a room filled with distorting mirrors, a public toilet imagined as a reflective capsule and a series of colour-changing lights.

The tunnel extends 750 metres through the Kiyotsu Gorge, which has a reputation for being one of Japan's most impressive chasms. It offers four viewpoints facing the gorge's unusual rock formations, including one that faces right down into the canyon.

The most striking of MAD's installations is located at this final point. Intended as a celebration of water, Light Cave is a tunnel-like space with a pool of water covering its floor and mirrors lining its barrel-vaulted walls and ceiling.

The reflections in the floor give the false impression that the space has a round outline, while the reflections in the walls bring new colour into the tunnel.

MAD describes the installation as bringing "both a lightness and stillness into the once dark, dewy tunnel, invoking a feeling of everlasting solitude".

"A shallow pool of water gently ripples with the breeze of the wind. In turn, the images of the gorge reflected from the cave cast themselves onto the water – an infinite illusion of nature," said the design team, which is led by studio principal Ma Yansong and architect Yosuke Hayano.

Located at the entrance to the tunnel is another of the installations, inside a wooden structure that also hosts visitors facilities. Called Periscope, it is a round first-floor room containing a hot-spring foot spa.

The space is lined with wood and features a circular aperture in the roof. This opening is framed by mirrors so, when guest bathe their feet in the water, they can enjoy reflections of the surrounding nature.

Another of the five installations is located within the tunnel itself, to represent the element earth. Called Expression of Color, it comprises a series of coloured lights that are accompanied by "mysterious music".

MAD said this area is designed to create "a subtle but dynamic ambiance that sparks a certain curiosity of the unknown for those who are wandering through".

Invisible Bubble is an installation located at one of the first lookout points in the tunnel. It comprises a "capsule-like structure" containing a public toilet. The viewpoint here is covered in a metallic film that allows views out but not in, to give the user privacy.

"Providing a quiet escape, a place of solitude, it is an intimate space that finds itself in an openly public setting," said MAD. "It puts into question how people react when they think that no one is watching – an ideal corner of contemplation."

Next up is The Drop, a room covered in small convex mirrors with red lights hidden behind them. This space is intended to reference the fire element.

"As one looks into these convex mirrors, backlit by fiery, red light, they experience an alternative connection with nature – one that is at once mysterious and warm," said MAD.

Ma Yansong founded MAD, which is ranked 56 on the latest Dezeen Hot List, in 2004. The studio has recently won a number of high-profile commissions, including the Lucas Museum of Narrative Art under construction in Los Angeles, USA, and the Harbin Opera House in China.

It is not the only big name involved in this year's Echigo-Tsumari Triennale, which was set up to reviving cultural interest in the declining region. Other particpants include Dominique Perrault, Antony Gormley, Leandro Erlich and Carsten Höller.

The triennale started on 29 July and continues until 17 September.