Dangerous Ground is a large area in the southeast part of the South China Sea characterized by many low islands and cays, sunken reefs, and atolls awash, with reefs often rising abruptly from ocean depths greater than 1000m. The area is poorly charted, making it exceptionally dangerous to navigate – the major Singapore-to-Hong-Kong routes go well to the west and east of the area. The Admiralty Sailing Directions give the following warning regarding navigation in this area: The water is a usually greenish-blue and is transparent to depths of 24–42 metres (79–138 ft) on clear days.