It works. I can find underground tile lines with 2 pieces of #9 wire; one held in each hand. Cut them about 3 foot long, bend one end in a L shape for a handle.
Just hold them loosely straight out and walk. When they cross you are over water or a tile line.
Walk a bunch of times in a grid pattern with a can of spray paint to mark, and its pretty accurate.
Not sure why it works; guessing some type of magnetic or electrical reason.
Make an open fist, stick the short arms into each hand and walk.
The wires cross. Back up and they open. Forward, they cross again.
Strange but true.
It works because of electromagnetic fields the exist naturally around different molecular structures. I use metal clothes hangers straightened out and bent into Ls. Hold two together in one hand. When you cross over changes in the EM fields the metal rods will react to change.