<P> Oceans are fringed by smaller, adjoining bodies of water such as seas, gulfs, bays, bights, and straits . </P> <P> The mid-ocean ridges of the world are connected and form a single global mid-oceanic ridge system that is part of every ocean and the longest mountain range in the world . The continuous mountain range is 65,000 km (40,000 mi) long (several times longer than the Andes, the longest continental mountain range). </P> <P> The total mass of the hydrosphere is about 1.4 quintillion metric tons (7018140000000000000 ♠ 1.4 × 10 long tons or 7018150000000000000 ♠ 1.5 × 10 short tons), which is about 0.023% of Earth's total mass . Less than 3% is freshwater; the rest is saltwater, almost all of which is in the ocean . The area of the World Ocean is about 361.9 million square kilometers (139.7 million square miles), which covers about 70.9% of Earth's surface, and its volume is approximately 1.335 billion cubic kilometers (320.3 million cubic miles). This can be thought of as a cube of water with an edge length of 1,101 kilometers (684 mi). Its average depth is about 3,688 meters (12,100 ft), and its maximum depth is 10,994 meters (6.831 mi) at the Mariana Trench . Nearly half of the world's marine waters are over 3,000 meters (9,800 ft) deep . The vast expanses of deep ocean (anything below 200 meters or 660 feet) cover about 66% of Earth's surface . This does not include seas not connected to the World Ocean, such as the Caspian Sea . </P> <P> The bluish color of water is a composite of several contributing agents . Prominent contributors include dissolved organic matter and chlorophyll . Mariners and other seafarers have reported that the ocean often emits a visible glow which extends for miles at night . In 2005, scientists announced that for the first time, they had obtained photographic evidence of this glow . It is most likely caused by bioluminescence . </P>

The hydrosphere composes approximately what percent of the earths surface