Outside of novels, movies and video games, pirates didn't bury their spoils: They spent it. The only treasure to be found in stories like these comes out of the pockets of the gullible souls who search for it.
Incidentally, and yet again, Wikipedia is a much better source than the original submission:
http://en.wikipedia.org/wiki/Oak_Island
On Wikipedia they tell a really funny story about a stone that was allegedly found in the pit, with "symbols" that turned out to be a "cipher message" that alluded to Tons of Money lying somewhere below. The stone later disappeared, of course. Shucks. Why does that always seem to happen to these mysterious cipher stones that only one person can read? Maybe some angels took the stone up to heaven so that we wouldn't find the Holy Grail. Or something.
Wikipedia also alludes to an important hypothesis, itself dating back to 1911: The "money pit" was a natural sinkhole, the early excavators' discoveries of "man-made platforms" were the product of starry-eyed optimism, and (conveniently!) the site eventually became so torn up by treasure hunters that nobody will ever be able to reconstruct what was originally there. And so the game of telephone will continue into the indefinite future.
Every 10 feet was a layer of logs that was left over from the construction.