Join us as we explore the vast and varied world of the arch, one of the strongest and most versatile structures made by man. Deceptively simple, an arch can support tremendous weight because its structure is compressed by pressure, and it provides a much more spacious opening than its predecessor–post and lintel construction. Although ancient Egyptians and Greeks experimented with the arch, the Romans perfected it. Medieval Arabs incorporated it into stunning mosque architecture, soon followed by Europe's great medieval churches. In the 19th and 20th centuries, the steel arch became a favorite of architects and structural engineers. Dam builders employed it horizontally, using the water behind the dam to provide the pressure to compress it. And tomorrow, the arch will continue to serve mankind in every form–from nanotechnology to domes on Mars and beyond.