The Kim Chuan Depot, located in Hougang, Singapore, consists of an underground rail depot with a multi-storey bus depot located above it. The depot provides maintenance, stabling and operational facilities for the Mass Rapid Transit’s (MRT) Circle Line. When it officially opened in March 2009, the Kim Chuan Depot was the world’s largest underground depot in its class, with a total area of 11 hectares.

The Kim Chuan Maintenance Depot is the world’s first underground mass rapid transit facility of its kind and one of the largest depots in the world.

More than 0.62 miles (1 kilometer) long and 492 feet (150 meters) wide at mid-point, it is situated 66 feet (20 meters) below ground. It serves as a stabling and maintenance facility for up to 70 three-car driverless trains operating on the Circle Line and a future Eastern Region Line.

To optimise land use, the expanded section of the Kim Chuan depot will also have an above-ground facility to house 550 buses.

According to Wikipedia