Hadrian's Wall marks the Roman Empire's northernmost boundary, and at one point is less than a mile from today's border between England and Scotland. Ari Shapiro/NPR hide caption

itoggle caption Ari Shapiro/NPR

Britain's Prime Minister David Cameron speaks at a news conference in London on Friday after the U.K. raised its terror alert level. Paul Hackett/PA Photos/Landov hide caption

itoggle caption Paul Hackett/PA Photos/Landov