One of the advantages of a buried tank is it's not going to BLEVE ( http://en.wikipedia.org/wiki/BLEVE ) from being overfilled due the suns warmth. If it were to develop a leak you might not be able to smell the propane since it's heavier than air.
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contractor says that he has his propane tank underground and that they have some kind of system in place where the tank is surrounded by these air filled bags(?)and is checked once a year.
...a mystery to me....