subterranean
One good word would be subterranean,
The word could be seek or some version of seek.
Start
Sloe.
One option is swear.
Source
scabies
Subway
An underground tomb or burial place beginning with cat is catacomb/s.
The word you're after is sweepstake
The word that is a synonym of "look hard" starting with S is "scrutinize."
Seta is a 4-letter word for hair or bristle starting with S.
what word starting with s is a synonym for technology? playful