Help identify (abandoned?) buried line
It turned out to be abandoned electrical. Photos below (cell phone for scale)

DigSafe came out and confirmed it was not a City service (gas, hydro, transit, water, etc.) They checked out the line and agreed it was strange it was so shallow; definitely not one of theirs. So we excavated a little more and eventually realized it is not a pipe, but a sheathed cable (with the sheathing completely deteriorated). Tested for voltage (there was none).

So I think this must have been how electricity used to get to the garage. As I mentioned, the garage was completely rebuilt a couple of years ago, and they put in a proper aerial connection at that time.

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