Kolkata is located over the "Bengal basin", a pericratonic tertiary basin.[64] Bengal basin comprises three structural units: shelf or platform in the west; central hinge or shelf/slope break; and deep basinal part in the east and southeast. Kolkata is located atop the western part of the hinge zone which is about 25 km (16 mi) wide at a depth of about 45,000 m (148,000 ft) below the surface.[64] The shelf and hinge zones have many faults, among them some are active.