I have a terrestrial planet that is similar to Mars in many ways, a desert-like surface, thin atmosphere, and inhospitable to large flora and fauna as we know it.
This planet has ceased tectonic activity yet still has a weak active core producing geothermal power. The planet also has a wide array of interconnected caverns, tunnels, and underground pathways that are spread across the planet but don't have any openings to the planet's surface.
The caverns were largely created through excavation by organisms (they could create tunnels through rock) that inhabited the planet before going extinct, and the holes have just been covered up over time by chance.
Is it possible that this cave system could hold an atmosphere similar in pressure to that of Earth's, for a window of 100 to 200 million years?