A lava tube is just what it sounds like; a cave created when the surface of lava hardens but continues to flow underneath. When that trickles to a halt, the cave is left behind.
These remaining caves exist on Earth and high-resolution pictures taken by interplanetary probes inferred lava tubes on Mars and Luna by observing linear cavities and sinuous collapse chains where the galleries cracked.
A new paper measured the size and gathered the morphology of lunar and Martian collapse chains (collapsed lava tubes), using digital terrain models obtained through satellite stereoscopic images and laser altimetry taken by interplanetary probes.