noun, noun or participle which takes 'suru'
capsizing and sinking
Refers to a vessel overturning and going under water. Often used in nautical or disaster contexts.
その船は嵐で覆没した。
The ship capsized and sank in the storm.
沈没 means 'sinking' in general, while 覆没 specifically includes the overturning (capsizing) before sinking.
Compound of 覆 (overturn) and 没 (sink). The exact historical derivation is uncertain, but the meaning is transparent from the kanji components.