subterranean
The word could be seek or some version of seek.
Sloe.
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Source
One option is swear.
scabies
Subway
An underground tomb or burial place beginning with cat is catacomb/s.
The word you're after is sweepstake
A word starting with "s" that means ghost or apparition is "specter." It refers to a phantom or spirit that is often perceived as haunting a particular place or person. Specters are often depicted in literature and folklore as eerie or unsettling figures.
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.