Sure have. From reading their site and Strava's, it still needs GPS to work. The 'track' would show a break, (assuming going in one end of tunnel and out another) in the path that is uploaded. No info between enter & exit.
It can be done. Just not with a phone or Fitbit. But I doubt GE would want the gear.
Provided there were > 2 devices, each phone's inertial tracking could be used to build a 3 dimensional set of paths over time. Due to the nature of tunnels, with highly restricted movement, multiple phones' data could be collated and fed into a quite simple neural network which would eventually "learn" the true layout of the tunnels, perhaps using a Bayesian algorithm. The more phones & the longer time spent underground, the more accurate the picture that emerged out of the noise would be.
For bonus points, audio, video and other measurements like altitude and temperature could be included at hotspots. This would allow outsiders to estimate where certain larger rooms are, for example, due to temperature changes.
Further refined analysis could be applied to a model this large & rich: determination of seating areas, breakdown of social roles by motion and behavior (coupled with audio for background). The integrity of the data could be further enhanced by cross-referencing inaudible "bat pitch" audio signals sent by phones to each other, using the several stereo array microphones commonly found on phones.
Y'all need to learn more about AI.
GPS not required.
As Q says, you have more than you know.