Technology for Treasure hunting

Today Liam and Richard went on an adventure. They downloaded some geocache sites into the GPS and went off to find them. The sites were all within about 1/2 a km of our place, near the boat pond and beach at the end of the road. They came back having found two of the three they set out looking for. One of the favorite instructions was to watch out for muggles (ordinary folk, not looking for geocaches) and not disclose the cache to them.