GPS alternative taps cosmic rays for underground or underwater navigation

I would guess never. The iPhone derives a lot of it's value from communicating with servers all the time. High-accuracy timekeeping just isn't valuable.

I find this quite surprising. We use chip-scale atomic clocks in our equipment, and while expensive, it hardly seems out-of-reach for the type of customers who might be interested in underground surveying (mines, for example).

I guess they are referring to chip scale atomic clocks, which are apparently already a real thing. Seems like scaling up manufacturing to commercialise is the current challenge, so maybe a few years away?

https://en.wikipedia.org/wiki/Chip-scale_atomic_clock

I'd be moderately surprised if it was necessary: atomic clocks are only really king of time stability over longer time periods (days/months). If this just needs to stay in sync over the course of a few hours (after syncing outside with GPS, for example), then it's much more plausible to do it with a high-precision crystal or silicon MEMS oscillator (SiTime has already become a big player in the phone market because of their low power consumption for the precision they offer, and they have some really impressive higher-end offerings that wouldn't currently fit but perhaps could be made to).

Microchip sells SA65, which is 40x35mm and retails for about $3000.

It doesn’t neeed to be real-time though: for some applications it’s fine to just record then establish after the fact where exactly the receiver traveled.