I imagine in soil with any amount of rocks at all, ground penetrating radar and seismology are going to detect the rocks better than they would worms. I wonder if the worms make enough "noise" or vibration to be detected passively? I imagine not, but that seems like it has at least as much potential as the active detection methods for this case.
Speaking of which I was wondering, a spider building a web is probably making some interesting noises (the web vibrations), I wonder if it's possible to record that.
Turns out large microphones are more sensitive, in general. Like here, they use a 1" microphone, I believe, to record a snail munching lettuce and a caterpillar tap dancing: https://www.youtube.com/watch?v=zJH5n_HdNkA
Remardkable. So it's a very sensitive condenser mic, and a very low noise preamp. I wonder if this would be enough for the spiderweb. Glass and lettuce are a lot louder, I think. And you'd probably want it physically coupled to the web.
How about feeding worms a radioactive tracing agent to follow their trails?
Also, PSA from my high-school biology teacher that has been living rent-free in my head so far: If you intend to eat earthworms, letting them feed on coffee grounds helps clean their intestines so you don't get sand in your teeth.
It's like a snake worm thing.
https://en.wikipedia.org/wiki/Caecilian
And apparently they produce something like milk to feed their young. Wild!
https://en.wikipedia.org/wiki/Caecilian#Milk_provisioning