prozack is correct, would also like to add that if you bury copper wire with pipe and leave end accessible , in the future a metal detector type of device could accurately trace the entire run. oh and some probe rods are fiberglass

if you remember the general area and the pipe is not filled with dirt?You can try a slender metal rod and poke around the area.If you hit the pipe youll feel it and hear it hitting plastic.This could take a long time depending how deep its buried and how long the pipe is,The pipe thickness and if its buried horizontal or vertical? good luck

start digging.