As part of my hex contribution to the Tenkar's Landing crowdsource project, I elected to throw a couple of volcanoes into my hex - one dormant (but still geothermally active) and one extinct. The area around the extinct volcano is riddled with lava tubes, which are in the process of being explored by a local band of lizardmen seeking artifacts of their species.
They are not uncommon in volcanic areas - both stratovolcanoes and shield volcanoes. My semi-local example is along the slopes of Mt. St. Helens. Tubes may be quite cavernous, or may pinch down to crawl-spaces or nothing, as repeated flows fill in the cavity.
Access may be through a 'typical' cave entrance, or via a roof collapse.
The challenge with a lava tube system is that the passages tend to be linear, with very few branchings or chambers. Characters could end up exploring multiple dead-ends and in-and-outs before encountering anything of note other than whatever cave fauna has moved into the chambers. However, as a natural tunnel (many may be over a mile long) it can interconnect two areas. An interesting possibility, especially if the lava tube is an unknown quantity, or perhaps acts as a bolt-hole from some residence or redoubt.
So although the tube or tube system doesn't have the tortuosity of a dissolution cave (ex. limestone), it still has the potential for some interesting features, geometries and interconnections.
|The "Meatball" inside Ape Cave, Mt. St. Helens|
|Lava "gutter" and tube-within-a-tube|
|Branching lava tube or side-tunnel|
|Page with more features and definitions|
For the crowdsource exercise, one or more tubes will access ancient lava chambers (this may be more artistic licence then scientifically accurate), where there is the potential to find interconnection over multiple levels, ancient workings and macguffins. An event, such as an earthquake or landslide, may expose a heretofore unknown tube or passageway, or clear a formerly blocked conduit. Now to create a few levels for some populating...
|Looks like something out of Giger|