bottom out has no recorded synonym. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is top out.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
hit the ground
“the car bottomed out where the driveway meets the road”
Synonyms, antonyms and the broader and narrower relations for bottom out come from Princeton University’s WordNet. Full sources and licences.