The tunnels connected to the "hall of heads" and David's lair have similarities in structure to a place that actually exists here on earth.
You have certainly seen these pictures before:
This is the Fingal's Cave in Scotland.
Here's a quote from Wikipedia:
Fingal's Cave is a sea cave on the uninhabited island of Staffa, in the Inner Hebrides of Scotland, known for its natural acoustics. The National Trust for Scotland owns the cave as part of a National Nature Reserve. It became known as Fingal's Cave after the eponymous hero of an epic poem by 18th-century Scots poet-historian James Macpherson.
Here is what interests us:
In all these cases, cooling on the upper and lower surfaces of the solidified lava resulted in contraction and fracturing, starting in a blocky tetragonal pattern and transitioning to a regular hexagonal fracture pattern with fractures perpendicular to the cooling surfaces. As cooling continued these cracks gradually extended toward the centre of the flow, forming the long hexagonal columns we see in the wave-eroded cross-section today. Similar hexagonal fracture patterns are found in desiccation cracks in mud where contraction is due to loss of water instead of cooling.
After you've seen the pictures, take a closer look at the tunnel from the movie:
Now, see the floor?
Check this out:
Again from Wikipedia:
Large masses must cool slowly to form a polygonal joint pattern, as here at the Giant's Causeway in Northern Ireland
A definition of basalt:
Basalt (pronounced //, // or //) is a common extrusive igneous (volcanic) rock formed from the rapid cooling of basaltic lava exposed at or very near the surface of a planet or moon. Flood basalt describes the formation in a series of lava basalt flows.
Geologically speaking, that tells us a lot about the tunnel. If I was to ask Fifield about this whole thing, he'd be telling me this place is all volcanic.
Just a little something I wanted to share:
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