Hawaiian Lava Tube

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Summary

Description	Photo by Dave Bunnell A lava tube on the island of Hawaii, taken just above a lava falls. The floor is cauliflower pahoehoe, a rougher form of pahoehoe. Note the tree roots coming in from the ceiling. Lava tubes tend to be fairly close to the surface.
Date	2006-11-03 (first version); 2006-11-03 (last version)
Source	Originally from en.wikipedia; description page is/was here.
Author	Original uploader was Dave Bunnell at en.wikipedia
Permission (Reusing this file)	CC-BY-SA-2.5.

End quote from Wikipedia under the heading Hawaiian Lava Tube:

I have walked or climbed like this through many lava tubes in Lava Beds National Monument on the Eastern Side of Mount Shasta. There are also various Lava tubes on all sides of Mt. Shasta. Some are safe, some have collapsed and some are partially collapsed. If you go in any Lava tube anywhere on earth it is not necessarily safe because often the rock is very brittle almost like glass and so for this reason one must be very careful entering any lava tube anywhere on earth. However, the more people that have been there the more likely it is relatively safe to enter.

Lava Beds National Monument - National Park Service