Almost all the levers and pulleys reveal those areas at the beggining. Test by sending a Slig down, if necassary. If it doesn't scream help, then it isn't a bottomless pit, and so there's a secret area. If you know 'em anyway, life will be easier without the Sligs down there. Just a suggestion when you're 'meant to' send them down there as part of the default route.
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