Back when i superintended new home construction, I had many occasions to find buried water and sewer lines. The monuments left by the developer had disappeared and we needed to do the house connections. I watched my plumber use variour bent rods to dowse for the pipes, and was skeptical. UNTIL he put the rods in my hands, and I felt them turn!
Then I talked with a friend of mine who confided, that if there are various pipes( Storm drain, gas, water, sewer) in an area, you will find the one that you are looking for, and ignore the others. He also had a method for determing the depth that was accurate to a foot or two for pipes buried to about 10'.
I haven't used the skill for several years, but still have some bent welding electrodes in my truck tool box.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags