When a cavernous sinkhole opens up in a bustling city, a claustrophobic geologist leads a team of experts down to investigate, but when the sinkhole collapses, they find themselves trapped underground? and they?re not alone.

I like your premise - also the idea of changing genre and story part way through is really intriguing. ?Do you also change main character? ?Personally I love it when a hero bites the dust and another person steps in LOL. ?My thoughts are:

• As you're aiming to flip the story part way through - do you need two inciting incidents? ?One for the first half - save the skyscraper... and one for the second half - get out alive...
• When the sink hole actually collapses, you'd need to make sure that what was threatened in the first half (e.g. the skyscraper) either doesn't actually collapse, or it is that which collapses and traps them underground?
• "and they're not alone" is a bit vague - it could mean many things... other people, another city, dinosaurs! I like the something wants them dead route.

Since the bulk of the story takes place in the earth, you can actually drop the first part of the logline. Just start with the sinkhole and it will save logline space for other things.

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"When a claustrophobic geologist is trapped with his team while studying a sinkhole, The bookish professor must lead the team out from the depths of the earth all while, something in the dark seems to want them dead."