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Whirlibird is totally correct! If they want it bad enough they’ll find it. If enough technology is available, as is the case many with major urban LEO’s, Thermal imaging, ground penetrating radar, and sonar arrays can can uncover anything you can get to without a submarine. From a practical SHTF standpoint “TC 31-29/A Special Forces Caching Techniques” is an excellent reference that covers what needs to be considered in hiding “stuff”. There seem to be several versions of 31-29A on the web for download. My copy is 25 pages long and goes into some considerable detail. One I looked at was much more general and only 5 pages, so look around and find the long version.