Advanced civilization on planet with atmosphere only in large sinkholes

I want to have an Earth-sized planet (with similar gravity) with an extremely low-density atmosphere (comparable to Mars) at most of its surface. Complex life, including an information-age civilization, lives in enormous sinkholes many kilometers across, some up to 100 km across. Due to the lower elevation, the air pressure and composition is comparable to Earth"˜s. The sinkholes are flat on the bottom and have vertical-ish walls, so they're shaped like carved-out cylinders. The planet has many such giant sinkholes, and they're connected by underground rivers.

Is this realistic?

posted almost 6 years ago

taylor swift‭

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