Learn Russian: Почему этот тяжёлый камень смог всплыть? - Why was this heavy stone able to float up?

The base verb плыть means to swim or float in a single direction. Adding the prefix вс- (a variation of the upward-moving prefix вз-) adds the meaning of upward movement. Together, всплыть means to float up or to surface.

Why is the prefix spelled вс- here instead of вз-?

The prefix вз- changes to вс- when it attaches to a root starting with a voiceless consonant. Because the letter п in плыть is voiceless, the prefix becomes вс- to make the word smoother to pronounce.

Why is the perfective verb всплыть used here instead of the imperfective всплывать?

The sentence focuses on a specific, successfully completed result: how the stone actually managed to reach the surface on this one occasion. After смог (managed to / was able to), we use the perfective aspect to emphasize that the action was successfully achieved.

Why is the verb смог in the masculine form?

Смог is the past tense form of смочь (to be able to), and it must agree with the subject of the sentence. The subject is камень (stone). Even though it ends in a soft sign, камень is a masculine noun, so it takes the masculine verb form.