Rather than relying on markers, I think the other earlier suggestion of making the pipes/ducts findable later is the way to go. When I had 800 feet of new water main put in, using 2 inch PVC, I just attached an insulated copper wire to the outside of the pipe. The above mentioned metallic tape would work as well, as would using metal pipes.

Then you just need a general location (which GPS could give you) and a metal detector to find everything.

Seems much easier than using markers and/or maps that could get lost over time.