A dome-like heightening in a lava tube’s ceiling is a cupola. These are sometimes found in lower-level passages beneath windows that have sealed off, as is likely for the first image. Or they may be caused by breakdown from a ceiling that has later been coated by a subsequent lava flow that filled the passage. The structure in the second photo, from a deep lava tube in Vietnam, is more puzzling but may be a second skylight in the making via breakdown collapsing along some zone of weakness in the lava matrix.

AUTHOR: Dave Bunnell