Meaning: This sentence describes a hiker's exciting discovery of a hidden waterfall during his adventurous journey through the mountains.
Choice of Words: Stumbled upon is a verb phrase that means to come across something unexpectedly. Hidden is an adjective that describes the waterfall. Waterfall is a noun that refers to a natural flow of water over a steep drop. Trek is a noun that refers to a long and challenging journey. Mountains are nouns that refer to large natural elevations of land.