We can't see in the dark. That's why it's called "the dark." And that can be inconvenient -- if you're walking around, you'll bump into things, for example. So throughout human history, we've tried our best to bring light into dark places. Torches, light fixtures, flashlights, and increasingly, cell phones have all been our aides in navigating the world when the sun has gone down and we still need to figure out where we are and what to do. And also, what time it is.

Watches and now phones with light-up screens mean that you can always know what the time is, even if there's no other light source around. That's nothing new; LED clocks have been around since the 1970s and we've had plenty of other ways to similarly illuminate a clock face for decades. But a century ago, being able to tell time in the dark. . .  that wasn't so simple. That's why Undark was such an incredible creation. As explained in the advertisement seen here, manufacturers of timepieces could "apply [Undark] to the dials of watches and clocks" and never again will you have to "wonder vainly what time it is because of the dark." Undark -- this miracle material -- "shines in the dark."

And it was very popular. And, unfortunately, deadly. Take a look at that ad, and you'll quickly see why: Undark was made of radium.

Radium, as we now know, is a radioactive element that causes cancer in humans. In the very early 1900s, though, the dangers of radium were not well known, and to make matters worse, some even believed that the substance provided some medical benefits. In the late 1910s and into the 1920s, some energy drinks contained radium and proudly advertised that fact. For example, there was a drink called Radithor, which, as CNN notes, "was simply radium dissolved in water." it sold for about $1 at the time -- about $15 today -- and in large amounts: "its manufacturer claimed the drink not only provided energy but also cured a host of ailments" and also made men more virile in the bedroom. None of these claims later proved true, of course; rather the drink ended up claiming the lives of many of its biggest fans.

But the "radium is good!" trend spread well beyond energy drinks. The manufacturers of Undark similarly saw radium as something worth promoting; the ad copy starts with the phrase "The Power of Radium at Your Disposal." Unfortunately, to make the glow-in-the-dark watches, that power had to be harnessed with precision -- and this is what that looked like.

In the early 1920s, some of the Radium Girls started developing symptoms like fatigue and toothaches. The first death occurred in 1922, when 22-year-old Mollie Maggia died after reportedly enduring a year of pain. Although her death certificate erroneously stated that she died of syphilis, she was actually suffering from a condition called "radium jaw." Her entire lower jawbone had become so brittle that her doctor removed it by simply lifting it out.