Undarra is the site of the longest lava tubes in the world. 64 tubes have been located and 9 of them are open to visitors. They are quite fascinating.
They all formed from lava rivers slowly oozing from the Undarra Volcano and spreading across the plains. The outside solidified and the molten lava inside kept moving - leaving the tubes behind.
The tubes were then covered with silt - but the roof in many of them has collapsed - creating holes which allow access to the tube below. :)
