Los Angeles is a city filled with famous faces and places, though perhaps none more so than the historic Hollywood sign that hangs in the hills above it all. The 50-foot white HOLLYWOOD block letters have been immortalized in popular culture the world over, standing in as a backdrop for an actual place — that being Hollywood, the neighborhood directly beneath the sign — and a pop culture vision of the sprawling, glamorous film and television industry that keeps the world entertained. The sign has attracted weirdos, artists, knock-off T-shirt makers, and tourists for 100 years — and for a time, it even had its very own swimming pool.

Though little known now, there really did used to be a fully functional, in-the-ground swimming pool set into a flattened parcel of land atop the Santa Monica Mountains peak that holds the Hollywood sign. The pool and its surrounding cement patio can still be seen in archival photographs from the 1940s, and its azure waters played a surprisingly vital role in the birth of modern television.

First erected in 1923, the Hollywood sign actually began as the HOLLYWOODLAND sign, a massive advertisement built by a developer who was selling home parcels in the canyons below. The sign was meant to be temporary but became rather famous in the proceeding years as LA’s population continued to boom. Twenty years along, the letters were in pretty rough shape, though the land around it had only become more valuable — in part because seminal filmmaker Mack Sennett wanted to build an ornate mansion right at the mountaintop. Sennett had the peak behind the sign flattened to make way for the property, though it was ultimately never built.

Enter Don Lee, a pioneering radio man and early TV visionary. His company was at the forefront of the broadcast side of television and films (thanks to his extensive prior work in radio) and even began to build production facilities across the Los Angeles basin. There was just one problem: Early television broadcast signals could not traverse over hilly terrain, meaning the Santa Monica Mountains effectively cut Lee’s viewing audience in half geographically.

So in 1938, the Don Lee Company bought the flattened Mack Sennett land, and by 1939 had completed a production facility on site, including a 300-foot broadcast antenna — then the tallest in the world. The building not only had state-of-the-art equipment (and was the first West Coast station to broadcast a remote telecast in real time, the 1940 Rose Parade in Pasadena, California), it included one particularly quirky bit of excess: a swimming pool.

For the next several years, the production facility would broadcast film and television programming of all sorts, including a recurrent segment known as the Bathing Beauties, which featured women in modest two-piece swimwear actually soaking in the pool. But by the mid-1940s the facility — like the Hollywoodland sign beneath it — had fallen on harder times, as broadcasters turned instead to the much larger Mount Wilson in the nearby San Gabriel Mountains to expand their range.

In 1949, the Hollywood Chamber of Commerce gained control over the dilapidated sign, and cut off the LAND letters at the end, leaving the sign we know today. Well, mostly; the current sign was actually rehabbed again in 1978, with famous Angelenos like Gene Autry and Alice Cooper ponying up $28,000 apiece to buy all of the letters and keep the sign going. Today, the Hollywood sign is fenced off and inaccessible to tourists, surrounded by security features like motion sensors and roving helicopter patrols to keep vandals and lookie-loos at bay (though that hasn’t stopped some). As for Don Lee’s old pool, it’s long been asphalted over, though one important bit of the broadcast maven’s legacy still lives on there: the peak where the Hollywood sign sits is now named Mount Lee.