Purchased this house last week and the previous owners also did not know what it was. The pipe is heavy duty and set in concrete. There's water about halfway up the length of the pipe, but I haven't tested the depth.
UPDATE: it was definitely an early generation giant satellite dish. Not only did we find 2 buried coax cables running towards the house when we removed it, I've since spotted several other such remnant poles in backyards across town.