A series of 107 line-of-sight radio relay towers built from coast to coast across America after WWII. Connecting NYC to SF, these towers became the backbone of America’s wireless network. Built one by one east to west over seven years, the earliest eastern towers were made of concrete due to a shortage of metal after the war, and the later western towers are made of more economical and efficient metal frames. Similarly, towers built in hilly areas utilize the topography to gain elevation so can be short towers, while towers built on slowly rolling plains are taller to create their own elevation. Most still stand today, with very few serving their original microwave relay functions. Some hold cell phone boosters, others are overshadowed by their taller successors, and a few have been retrofitted for uses totally unrelated to wireless communication.