A lava tube, or tunnel is a natural conduit formed by flowing lava from a volcanic vent that moves beneath the hardened surface of a lava flow. If lava in the tube empties, it will leave a cave.
Lava usually leaves the point of eruption in channels. These channels tend to stay very hot as their surroundings cool. This means they slowly develop walls around them as the surrounding lava cools forming an insulating tube. As a result, this keeps the lava molten and serves as a conduit for the flowing lava.
In conclusion, Moun Etna is rich in volcanic tunnels, over 200 known ones.