Arched passages in the main tube show the classic lava tube shape. The floor was the crust on a former lava lake that fell inward as it drained from beneath.
<a href="//commons.wikimedia.org/wiki/User:Dave_Bunnell" title="User:Dave Bunnell">Dave Bunnell / Under Earth Images</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>