Between early wired networks and today’s fiber optics sat a system of microwave relay towers transmitting information from coast to coast across the United States. Built in the early 1950s, this line-of-sight network spanned the continent using zig-zag patterns to avoid signal overlap. It conveyed phone conversations and television signals from the era of the Kennedy assassination through the resignation of Nixon.

The towers were generally spaced 30 to 40 miles apart and can be hunted using old official charts or this unofficial Google Map. It was the largest network of its kind when it opened, and unique in relying on microwaves rather than transmission wires.

A few years back, photographer Spencer Harding got interested in the history of these towers and raised funds to shoot and publish a book about them. The Long Lines collects dozens of Californian towers from six weeks on the road spent documenting them (excerpts above).

Today, many of the towers are in disrepair or have been taken down entirely, but some are built into the very design of urban architecture. Others can be easy to miss among HVAC outcroppings, cell towers and other antennas but are possible to spot if you know what you’re looking for. In the countryside, old towers are often found adjacent to concrete bunkers (some are even occasionally for sale). At times, you can even catch a glimpse them in late night show backgrounds.

The microwave relay system was part of the AT&T Long Lines network, which included wire and cable connections. 99pi fan and Long Lines aficionado Corey Carlson explains that “the Long Lines network relayed analog data from one horn to another, with polarization of radio waves to expand the signal capacity.” Their “SUV-sized ‘horn’ antennas” were designed to focus radio signals out horizontally toward other towers and be durable, “resistant to the pressure wave from a nuclear blast.”

Obsolete in today’s world of fiber optics, satellites and wireless internet, many of the towers have been taken down or swapped out for cellular use. Some, however, serve as emergency backups in rural areas. And others, it would seem, are so built into the aesthetic of their associated buildings that owners have seen no reason to remove them.

For those looking to learn more about this technology, there’s a wealth of information on these systems — including images, maps, diagrams, magazine and newspaper clippings — to be found on Long-Lines.net.