Roads over lava tubes in Cheju Island, South Korea

There are 59 known lava tubes in the basalts of Cheju Island, South Korea. They have formed by either crusting or injection, and the structural properties of the rock over them vary with their formative processes. Two tubes are crossed by roads that stand on very thin rock arches. One road has been built on a bridging concrete slab, the other is unprotected, but both appear to be stable. A ratio of 3:1 for tube width to roof thickness appears to ensure integrity under normal engineering loads.