After Congress passed the Secure Fence Act of 2006, calling for 700 miles of new border barriers, former Homeland Security Secretary Michael Chertoff summoned his REAL ID powers to fence the banks of the San Pedro River in southeastern Arizona and the U.S. Air Force's Barry M. Goldwater Range to the west, among other places. (scientificamerican.com)
@JonathanReez: In a nutshell the agreement essentially puts forward that you can not have border barriers between Northern Ireland and the Republic of Ireland, and acknowledged (quoting the wiki page) «that a substantial section of the people of Northern Ireland, and the majority of the people of the island of Ireland, wished to bring about a united Ireland.» (politics.stackexchange.com)
It suggested an impermeable border barrier would disrupt this network, severing important corridors in Organ Pipe National Monument and Cabeza Prieta National Wildlife Refuge. (scientificamerican.com)