The Washington Post has begun a three-part online series on border walls titled "Raising Barriers." The first part, published yesterday, is available here.
According to the Post, there are currently 63 borders where states are separated by walls or other man-made barriers. Most of the world's barriers have been erected since 9/11. In Europe, the refugee crisis has triggered a new round of wall-building. In spite of this situation, the Schengen Area in Europe remains one of the world's most remarkable experiments in open borders.