What technology is required to detect an underground civilization?

Question

Suppose a world has huge underground caverns, capable of supporting large agricultural societies (something like described in Our Hollow Earth). What technological level, or better yet specific technologies, would the surface need to detect the underground dwellers? In the 20th century, geologists used nuclear tests to help map the inside of the earth; is that what's needed?

Let's assume that the cave people are not taking any extraordinary measures to hide, like magical acoustics to redirect the force of earthquakes and explosions and thereby hide from seismic mapping. However, they travel about the surface very rarely, so they're not going to just bump into the surface people. Bottom line: they topside people are not looking for underground civilization, but would investigate if they detected caverns totaling many cubic kilometers in volume.

Let's also assume that the troglodytes have no widespread industrial technology, so no massive release of CO2 or slag to dispose of... but if we can answer that too, bonus points!

Answer 1

Turn it into an answer

Shallow

• Humans are explorers and the unknown beckons us pretty readily...if there were surface entrances to these caverns, they probably would have been discovered relatively early in our history...either through dedicated explorers and spelunkers looking to venture where humanity hasn't before or attempting to get a cave network named after themselves (this would be more the exploration of a new continent or somewhere we haven't been before) or through curiosity, potentially even children, if these locations were nearby existing cities/towns/farms.

Deep

• Mining for various minerals has been around since recorded time. Depths vary of course, but we've been a long ways underground before AD.

• In the late 1800's, James White discovered the Calrsbad caverns. He saw a cave that was populated by an army of bats...any cave that can hold that many bats must be huge, and exploration began. Admittadely this is the white mans account, Native americans could have been down there far before we 'discovered' their land.

• Magnetic survey developed in the 1850's and was capable of detecting changes in magnetic fields underground. Caverns and caves could be detected as probabilities.

Deeper

• Passive Seismic surveys can see a good 250m below the surface pretty clearly and caverns above 1km are readily known. Passive is detecting the tremours that naturally occur. Average oil well is in the 3000ft deep range (1000 meters), and we are pretty adept at detecting those...by the late 1990's we had mapped the majority of oil producing lands to about 5000 feet.

• Hydrolic fracking operates at a depth of 5000 to 20000 feet (1000-5000 meters) and requires an active seismic reflection study..this is us causing the seismic ripples and watching what reflects them...We see layers of rock at this depth, and holes in those layers of rock are the oil pools we are looking for. Unsure if caverns is really feasible at this depth.

Deepest

• Russians have drilled to a depth of nearly 12km, Americans to about 10km (bertha rogers). Seismic reflection was able to detect pools at this depth, however Bertha rogers had to be abandoned after it struck molten sulphur around 9500 meters deep. We can see layers of the earth at this depth, but not much else.

Answer 2

Seismic detectors would detect these cavern's interference with the detection of earthquakes' S and P waves without any new technology.

Once you have them located setting off small charges on the surface would do for more detailed sonar work.

Answer 3

Depending on how deep these caverns are, and the composition of the earth above them, detecting such caverns could be done in many ways, several in our current technological state and some with more archaic means.

Near Surface

Let's assume they're only a few metres (10-15m, ~30-45ft) deep, and the earth above them is comprised of granite, or another solid stone. In such circumstances, caverns can be detected by our modern technology in the form of ground-penetrating radar, often used for archaeology or other low-depth scanning. Said technology was recently used for mapping the ground around Stonehenge, with surprising success. If modern humans were using this technology, they could stumble across underground caverns, and don't forget that humans are curious [citation needed], especially scientists. They might want to check out those caverns, leading to a great discovery.

Let's Go Deeper

What if people were mining in a region? Mines can be deep, the deepest reaching multiple kilometres down. Humans have been mining for thousands of years, with ever-increasing scale and depth. Occasionally, a mine could be built that would intersect one of your caverns. Or what if a mine collapsed, or a cavern near a mine collapsed during blasting, revealing the cavern nearby? People will potentially find it and, being people, want to check it out.

Lately, we've also been doing an awful lot of drilling (for oil, natural gas, etc.). Maybe a drill-well could intersect a cavern and, if science was curious enough, they'd want to check out this massive cave network they found (probably during the initial surveying of the drill site).

And what about something as simple as a spelunker going down a new cave entrance that no one else had ever explored? Earth's caves are big, and some are interconnected in strange ways! New passages can open all the time as the ground shifts or erodes. Eventually, a hole may open somewhere and humans could find their way in. Spelunking is a modern hobby, as far as I can tell, but it has still been practiced for decades. One does not need much tech to find an inhabited cavern completely by accident. Let's hope it doesn't turn out like it does in some of the movies, however.

There is also the possibility of deeper scanning, as noted here, which could easily spot a large cavern given time to refine the process.