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Because they rely on radio signals, GPS units generally do not work underground or underwater, but what about a plain ol' magnetic compass? Are the magnetic fields of earth present at all depths? Does water do anything to attenuate them?

Can I go spelunking with a compass to help me keep my bearings or even go through a train tunnel in a mountain (contrived example)? Do divers use compasses regularly?

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    Yes, a compass is standard equipment for a diver. – Pete Becker Apr 18 '16 at 15:57
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    Yes, unless you're near large quantities of iron ore… – gerrit Apr 18 '16 at 16:30
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    Also, possibly, if you're spelunking directly above a LHC? – Dan Henderson Apr 18 '16 at 22:32
  • Compasses should work under water, just be sure to get the waterproof model... – badatz Apr 19 '16 at 2:02
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    @Dan Henderson - What, a Large Hadron Collider? – nsandersen Apr 19 '16 at 18:14
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Compasses are good equipment both spelunking and diving. Even the deepest cave you could go to is still near the earth's surface, geologically speaking. The earth's magnetic field is also essentially the same under water as above.

If you are using a compass, what you need to be aware of is nearby magnets and large sources of iron. So if you were exploring an underwater metal shipwreck, it could throw off a compass, the same as if you are testing a compass in your house standing next to a metal stove or refrigerator. The same is probably true if you explored a cave with a very concentrated source of iron ore. If you store your compass with a strong magnet, you risk demagnetizing it so it won't work at all. But if you treat it properly, a good magnetic compass is going to be more reliable than any electronic ones I've tried.

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    One thing to be wary of underground: you will probably need a light to read the compass, so make sure that you either don't use a light which affects the compass or ensure the light is always held far enough away not to affect the compass. – Paul Lydon Apr 19 '16 at 11:15
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    Structures will alter the earths magnetic field, these days they even use geomagnetic positioning technology to map interiors of steel structure buildings. Fortunate thing about navigating underground is the magnetic field is still the same as when the cave was mapped, so the readings will be the same when you go through. – ShemSeger Apr 20 '16 at 17:43
  • What she said :) And when spelunking I would assume you need a light in anycase to see, but in scuba diving it is also a good idea to take a light as you never know how dark it can get :) – AquaAlex Aug 4 '16 at 6:28

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