giúp mình với Underwater is a term describing the realm below the surface of water where the water ________51_______ in a natural feature (called a body of water) such as an ocean, sea, lake, pond, or river. Three _______52______ of the planet Earth is covered by water. A ___53______________ of the planet's solid surface is abyssal plain, at depths between 4,000 and 5,500 meters (13,000 and 18,000 ft) below the _____54_________ of the oceans. The solid surface location on the ____55__________closest to the centre of the orb is the Challenger Deep, located in the Mariana Trench at a ______56_________ of 10,924 meters (35,840 ft). Although a number of human activities are conducted underwater—such as research, scuba diving for work or ____57_____________, or even underwater warfare with submarines, this very extensive environment on planet Earth is hostile to humans in many ways and therefore little explored. But it can be ___58___________ by sonar, or more directly via manned or autonomous submersibles. The ocean floors have been surveyed via sonar to at _________59________ a coarse resolution; particularly-strategic areas have been mapped in detail, in the name of detecting enemy submarines, or aiding friendly ones, _____60___________ the resulting maps may still be classified.