(Bob Milnes/Stars and Stripes)

Salisbury Plain, England, March 1967: Years before the combined wear-and-tear of tourism and nature prompted restrictions on access, visitors of the human and canine varieties were free to stroll through Stonehenge’s center circle. The mysterious structure — now a U.N. World Heritage Site — was built in three stages beginning about 5,000 years ago; its purpose has been a matter of scholarly debate for centuries.