Metal Detectors and treasure hunting are a fascinating hobby. If you are lucky, you will find interesting and probably valuable artefacts from ancient times.
Or go to your beach and collect coin and jewellery lost by sun tanners.

Here you will find Schematic Circuit Diagram for DIY and further information on Metal Detector
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Nice Microcontroller operated Detector with PCB	home.skif.net/~yukol/MetalE.htm
Complex detector with Pulse Induction	metaldetector.bratsk-city.ru/shematic/PI/twinloop.pdf