What makes an old site good for metal detecting?

Sometimes. Many ordinary municipal beaches allow metal detecting without a permit, some require a local permit, and others ban it outright during peak season or on protected sections. The right answer depends on who manages the sand. A city beach, a state park beach, a national seashore, and a wildlife refuge can all have different rules even if they sit next to each other. The correct field habit is to confirm the managing agency before you swing, not after you recover a target.

No. In U.S. national parks, metal detecting is generally prohibited. National Park Service rules treat treasure hunting and the disturbance of cultural resources as incompatible with park protection. That prohibition applies even when a detectorist only intends to look for modern coins or jewelry, because the activity still risks damage to archaeological material. If the ground is under National Park Service jurisdiction, the safe assumption is that your detector stays off.

State parks do not have one national rule for metal detecting. Some state park systems allow it only in designated recreation areas, some require a permit, and many ban it because of archaeological or natural-resource concerns. The accurate answer is that state-park rules are park-system specific and sometimes even unit specific. If the site has historic structures, protected dunes, battlefields, or archaeological sensitivity, the rule is usually stricter than on a normal city beach or school field.

There is no single most valuable coin that is commonly found, because value depends on rarity, date, mint mark, condition, and whether the site realistically produces older material. In everyday U.S. detecting, silver coins, large cents, colonial coppers, and certain gold coins create the high-upside scenarios, but those are not common in a statistical sense. The useful answer is that site age and disturbance history drive value more than the detector model alone. Old ground produces the best coin upside.