Lava tubes on Mars are the prime target for human or robotic missions. They provide protection from radiation, surface conditions, and are thought to have stable internal temperatures. Lava tubes are also a high value scientific target because if there's life on Mars, it's most likely underground. Exploring lava tubes allows underground exploration without powerful digging or earth moving equipment. Lava tubes could also potentially easily be converted to human habitation by covering the cave walls in a liner and pressurizing the interior.
Question: Are there any past or present space agency plans to explore lava tubes and if so, what would such a mission look like?