A team of researchers suggests that lava tubes on both the moon and Mars could provide the space, environment, and protection from radiation needed to accommodate living things, humans included.

Scientists have long been investigating the habitability of other celestial bodies, be it to find signs of life or locate alternative homes for Earthlings, should our planet suffer irreparable and unlivable damage. 

Researchers with the University of Padova and the University of Bologna have found a naturally occurring underground feature that may serve both pursuits. 
The team suggests that lava tubes on the moon and Mars could provide the space, environment, and protection from radiation needed to accommodate living things, humans included. 
In a release about the findings, the tubes are described as forming, “when low-viscosity lava flows fairly close to the surface, developing a hard crust that thickens to create a roof above the moving lava stream.”
They can also develop when, “lava is injected into existing fissures between layers of rock or cavities from previous flows. The lava expands and leaves a huge network of connected galleries as it forces its way to the surface.” 
The release notes that the European Space Agency has already launched a program to familiarize "the astronauts with geological research they could carry out during future missions to the Moon or Mars, as well as to test technical and operational systems.”