A Graboid’s cylindrical shape has great strength to resist pressure (like a submarine does).  Also, moving deep into earth is not quite the same as moving deep into  water.  Various things can mitigate the amount of force the dirt applies.  Under the right soil conditions, a Graboid could theoretically go down several hundred feet.  The real limit to a Graboid’s ability to “dive” is that, like a sea mammal, it has to surface every so often to breathe.  It doesn’t have to actually break the surface because it can slowly pull air through a few inches of sand).  We don’t know how long one can hold its breath.

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