A peek through a 'skylight' into the interior of an active lava tube (USGS Photo/J. Judd) As islands created by volcanoes, Hawaiʻi is criss-crossed by countless lava tubes. These underground passageways, also known as pyroducts, are created by lava flows themselves and are capable of transporting great quantities of lava long distances underneath the surface. When the supply of lava stops at the end of an eruption, or if it gets diverted elsewhere, it leaves behind an empty cave.