Portions of a hike I did recently involved me going through tunnels that had some twists and turns. How would these work with an app like AllTrails or another GPS tracker?
As I understand it, assisted GPS is able to keep track of where you're at, even when satellites aren't available, by using an internal gyroscope. I feel like this means that most GPS trackers that record routes should be able to work, similarly, even when being unable to connect to satellites?
How would GPS trackers work when underground?
I have no personal experience with Alltrails, but the GPS apps which I have used in the past mostly did not cope well with GPS reception being spotty. Generally this resulted in strange erroneous routes being tracked, with large jumps in between points where the device had a GPS signal. How bad this is for you depends on a lot of factors:
The actual solution to your underground navigation problem is called Inertial Navigation, and is able to function completely without contact to any satellites. I am, however, not aware of any consumer-level products in this regard. Caving itself is a rather specialised undertaking, and navigation in/mapping of caves remains, as far as I know, rather involved and technical.
Assisted GPS is different from inertial navigation. It uses ground stations to transmit GPS almanacs and ephemeris for faster GPS start-up. There are also wide-area augmentation services such as EGNOS which work in a manner similar to differential GPS to improve accuracy by transmitting positioning signals from ground stations. Needless to say, both of these require reception of radio signals, which won't happen underground.
No handheld GPS receivers that I know of include an Inertial Measurement Unit (IMU), probably due to expense and conflict with the desire for long battery run-times. Usually an IMU has a tendency to long-term drift (they work by integrating accelerations) and benefit from at least periodic correction from GPS fixes. Without correction, you'll become increasingly inaccurate over time.
The best way to track your location underground is the traditional way, by plotting timestamps in pencil on your survey.
For making cave surveys, we also use traditional techniques - trigonometric survey created by measuring bearing, inclination and distance from station to station. There are electronic devices for the actual measurement, though ordinary compass, clino and tape are often preferred for their robustness.
Some people are experimenting with drone-based mapping, combining a drone's IMU and laser-scanning to generate point clouds which then subsequently become the "anchor" for the next bit of survey and so on. That's not in regular use for cave survey (and it does require enough room for a drone to manoeuvre, so only suited to large chambers) but is an interesting technology to watch. At present, it requires a lot of number-crunching back home to post-process the data, but it's conceivable that one day, real-time cloud-point surveying will become feasible.
GPS and similar technologies do not work at all underground, they can be blocked fairly effectively by dense tree-cover or being deep in a valley, even with clear views of the sky. Any thickness of rock will be impenetrable by the radio waves used to communicate with the GPS satellites.
Inertial guidance technology is improving, particularly with the advent of sports watches, but these generally need at least occasional reception from the GPS and/or regular stride patterns (e.g such as when walking, not something you often come across underground) for them to estimate the route with any degree of accuracy.
The simplest solution, and one that has been known and used for thousands of years is... a string/rope. This sort of thing is what cave surveyors actually use. Needs to be long enough for your purpose and strong enough that it doesn't snap easily. There's things like distance thread measures (e.g. this one) used in forestry that pull out a cotton thread up to 2.5 km (~1.5 mi), which would work pretty well for short non-critical distance - the devices aren't cheap, but I am fairly sure you could hold the thread spool in your hand or dangle it from a pack and have it unspool. The real risk here is that the thread breaks and you wander off without noticing that you are dragging a broken string, thereby separating the ends and leaving you none the wiser as to where you are.
As others have stated, the way to keep track of your position when you can't use GPS is inertial navigation, also known as dead reckoning. This uses various sensors, such as accelerometers, magnetometers and gyroscopes, which form an IMU (inertial measurement unit).
The issue is that it is by definition subject to drift (so with time the estimated position can get further and further away from the actual position), and that it's even more complex on a hand-held device such as a mobile phone as opposed to a car.
On a car, you can get (if the GPS is properly wired into the car system) two bits of information that are quite handy:
Each wheel turn, you add the size of perimeter of the wheel to your previous position in the direction of the orientation of the car.
This can give you a pretty accurate result. You will get slightly off if you turn a lot (or if the tire size doesn't match!), but the error should remain quite manageable.
If your GPS cannot get the wheel turns information (i.e. it's an add-on device, not wired into the car), then it get slightly more tricky, as you can't directly get a distance traveled. Instead, you rely on accelerometers, which will give you... acceleration (it's the only thing you can measure directly).
From acceleration, by integration, you can get speed: if you start at 0 m/s and measure a constant 1 m/s^2 acceleration, after one second your speed is 1 m/s, after 2 seconds it's 2 m/s, and so on. If acceleration drops to 0, then the speed remains constant.
From speed, you can get distance traveled via the same mechanism. Double integration means that your position is based on the square of elapsed time (x = x0 + 0.5.a.t^2 if acceleration was constant). This means that even a slight error in measurement of acceleration will quickly grow to become a quite large error in the measurement of distance travelled. To minimise error, you need to have the lowest possible error in measurement (better sensors), and perform measurements as frequently as possible (we're talking fractions of a second).
With this, it's still possible to get a decent estimate, especially as a car doesn't (usually) swerve around a lot, or have lots of variation in acceleration. The magnetometer being fixed in the car means that it can tell you quite precisely where the car is headed.
However this all goes awry when you consider a mobile phone. You have it in your hand. Try as you may, you will at the very least bounce it slightly left and right and up and down as you walk. You probably don't have a constant speed either, so you'll also impart a slight acceleration front and back. This means acceleration changes a lot in all 3 axes. Orientation as well.
This results in a lot more difficult measurement, will lots of errors, especially if you don't sample acceleration and orientation and rotation often enough. Modern IMUs will do that all inside the chip to try to get the best estimate possible, but that's a pretty difficult task.
Wikipedia reminds us that maths dictates that:
To get a rough idea, this means that, for a single, uncorrected accelerometer, the cheapest (at 100 mg) loses its ability to give 50-meter accuracy after around 10 seconds, while the best accelerometer (at 10 µg) loses its 50-meter accuracy after around 17 minutes.
And that's just based on pure acceleration, not even taking into account rotation of the phone.
Not all phones are created equal in that respect, and some will have better performance than others (because they have more recent, more expensive IMUs with less errors, higher frequency), but in any case, it won't (and can't) give you a precise position for a very long time: they need to be frequently re-synchronised to their actual position through other means (GPS or Wi-Fi/cellular trilateration).
To supplement @fgysin answer:
Modern smartphones contain enough sensors (acceleration, gyro, compass) to do inertial navigation.
These sensors are not of a great precision, so I think the integration errors will accumulate quickly.
Searching for "inertial navigation app" gives some results that pretend to do the thing.
I am really curious and will try some time soon.
