Scientists talk about the possibility of life underground on Mars, and I assume they are talking about microscopic life that can live in small gaps in the soil or rock. But is there any evidence that there might be large caverns under the surface of Mars relative to the human scale or even larger?
Then I wonder how could we even detect or map such subsurface features on Mars or other rocky bodies in the solar system such as Titan, Venus, or even the Moon. Could we use radar or maybe sonar for bodies with substantial atmospheres from an orbiting spacecraft? Are there other methods for detecting sub-surface cavities?