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Most of the planet's surface is made of water.
Literal
Planet [possessive-の] surface [topic-は] almost-all water [from-から] is-made.
Referring to Earth's surface. 表面 (ひょうめん) 'surface'. ほとんど ('almost, mostly'). ~からできている ('made of X') uses から to indicate the material/source from which something is made. This is a subtle particle choice — から rather than で is used when the material transforms into a new state.