How big could a system of caverns be without collapsing?

[Lren Zvsm]

In the horror novel, the Descent, by Jeff Long, Earth has a system of deep caverns so extensive that they constitute a virtual "sub-planet," a kind of partial lower deck in the Earth's crust, whose mutated human inhabitants have horns and decorate their bodies with extensive scarification. Hence the tagline: "Hell is real."

Ignoring the book's mutated humans, could such an extensive system of caverns theoretically exist on Earth or some other Earth-like planet about the same size as Earth?

 

[fresh_42]

What does

such an extensive system

mean in numbers? There are some pretty large systems on earth:

Mammoth Cave (KY) - $628\; km$ length (and counting?)
Voronya Cave (Georgia) - $2,200\; m$ depth (and counting)
Sistema Sac Actun (Mexico) - $333\; km$ under water (and counting)
Son Doong Höhle, Vietnam - $250\; m$ height (biggest dome)
Riko Riko Cave, New Zealand - $221,500 \;m^3$ (largest volume)

 

[Lren Zvsm]

Yikes! I don't have the relevant numbers. Jeff Long's book was written in 3rd person limited, with the protagonist coming from the surface world, and the protagonist didn't have time to explore the entire "sub-planet." Hence Long's sequels to "The Descent," which I haven't read. :-( I sure do need to look up Riko Riko Cave, though. Thanks.

 

[BWV]

Limestone is typically ancient sea beds, so limestone caves can get very deep, but there are limits

 

[some bloke]

Theoretcially, provided the cave roof is supported with regular columns, it should be physically possible to have a cave system spanning the entire earth. I don't know the geological standpoints for its realism, but certainly from an "is it mechanically and physically possible" standpoint, it would be doable.

 

[stefan r]

If re-worded slightly this question would be good for the Earth sciences thread.

Along the continental divide in North America between the Mississippi and St Lawrence water sheds there is a very large number of small lakes. They do not overflow the way mountain lakes or reservoirs overflow. Instead there is a fully interconnected water table. Sometimes the water flows through gravel when equalizing pressure. There are also extensive cave networks that divers explore. Diving is dangerous and it is can be difficult to measure position in flowing water. The famous cave networks like Mammoth cave are located in places where the water table dropped.

Where I live now, Eastern PA, we have frequent sink holes. It stands as evidence both ways. Clearly there is a lot of void space under the ground. But there are limits to the size since large voids collapse.

Wikipedia has this map. Its not "the entire world" but it is potentially a much larger area than the minimum needed for a civilization to exist.

I would expect planets that have lower gravity and tectonic activity to have larger cave systems. Saturn's moon Titan has an interconnected global water table (methane table?).

 

[Baluncore]

The depth of an extensive habitable cavern system is restricted to being above sea level. It must be well drained to avoid flooding. Cave water would drain to the surface above the denser sea water, so it must appear in a lake, a river or near the coast. The outflow would be contaminated by underground activity, unless it was well filtered.

If a cavern system was truly extensive, then it would intersect the surface in many places. There the cave air could exchange with the surface atmosphere to reduce CO2, which requires sunlight and photosynthesis. What do the troglodytes eat? The detection of air currents in a cave entrance would identify multiple portals.

Even without water, hydrostatic pressure increases with depth. That would limit the radius of curvature of walls, or the surface would spall into the cavity. At depth, the tunnels would need to be circular section and the caverns would need to be domed. Deviation of the arch would initiate a failure.

 

[White Hole]

I have imagined that perhaps the collision between the Earth and the Moon could create enormous continent-sized caverns, is this possible and could the caverns created last to this day?

 

[White Hole]

The depth of an extensive habitable cavern system is restricted to being above sea level. It must be well drained to avoid flooding. Cave water would drain to the surface above the denser sea water, so it must appear in a lake, a river or near the coast. The outflow would be contaminated by underground activity, unless it was well filtered.

If a cavern system was truly extensive, then it would intersect the surface in many places. There the cave air could exchange with the surface atmosphere to reduce CO2, which requires sunlight and photosynthesis. What do the troglodytes eat? The detection of air currents in a cave entrance would identify multiple portals.

Even without water, hydrostatic pressure increases with depth. That would limit the radius of curvature of walls, or the surface would spall into the cavity. At depth, the tunnels would need to be circular section and the caverns would need to be domed. Deviation of the arch would initiate a failure.

My caverns include very many portals to the surface (Don't ask why).

 

[White Hole]

Theoretcially, provided the cave roof is supported with regular columns, it should be physically possible to have a cave system spanning the entire earth. I don't know the geological standpoints for its realism, but certainly from an "is it mechanically and physically possible" standpoint, it would be doable.

My caverns have tons of those, naturally.

 

[Rive]

Regarding the original topic: maybe this would be a matter of interest, even in this serious case of topic-necromancy.

Over 3,000 km of underground tunnels — about the distance from Las Vegas to New York — and close to 1,500km of underground roads have been developed in the mine since it began operations.

So, in case somebody includes some fantasy xenobiology elements about gigantic rock-eating earthworms, the size of the maze can be practically indefinite.

I have imagined that perhaps the collision between the Earth and the Moon could create enormous continent-sized caverns, is this possible and could the caverns created last to this day?

Not likely. The bigger the cataclysm, the finer the gravel - and above a limit it's molten.
Something like that would rather collapse tunnels than create more.
Especially since an event of that scale would come with plenty of Earth'shakes.