How well do you know the periodic table? Our series The Elements explores the fundamental building blocks of the observable universe—and their relevance to your life—one by one.

Uranium took some time asserting itself. For centuries, heaps of it languished in waste rock piles near European mines. After formal discovery of the element in the late 18th century, it found a useful niche coloring glass and dinner plates. In the first half of the 20th century, scientists began investigating uranium's innate potential as an energy source, and it has earned its place among the substances that define the "Atomic Age," the era in which we still live. Here are some essential facts about U92.

1. IT'S THE HEAVIEST NATURALLY OCCURRING ELEMENT IN THE UNIVERSE.

With a nucleus packed with 92 protons, uranium is the heaviest of the elements. That weight once compelled shipbuilders to use spent uranium as ballast in ship keels. Were it employed that way now, sailing into port could set off defense systems.

Uranium was first found in silver mines in the 1500s in what's now the Czech Republic. It generally appeared where the silver vein ran out, earning it the nickname pechblende, meaning "bad luck rock." In 1789, Martin Klaproth, a German chemist analyzing mineral samples from the mines, heated it and isolated a "strange kind of half-metal"—uranium dioxide. He named it after the recently discovered planet Uranus.

French physicist Henri Becquerel discovered uranium's radioactive properties—and radioactivity itself—in 1896. He left uranyl potassium sulfate, a type of salt, on a photographic plate in a drawer, and found the uranium had fogged the glass like exposure to sunlight would have. It had emitted its own rays.

2. ITS TRANSFORMATIONS PROVED THE ALCHEMISTS RIGHT … A LITTLE.

Uranium decays into other elements, shedding protons to become protactinium, radium, radon, polonium, and on for a total of 14 transitions, all of them radioactive, until it finds a resting point as lead. Before Ernest Rutherford and Frederick Soddy discovered this trait around 1901, the notion of transforming one element into another was thought to be solely the territory of alchemists.

3. IT'S HIGHLY UNSTABLE.

Uranium's size creates instability. As Tom Zoellner writes in Uranium: War, Energy, and the Rock That Shaped the World, "A uranium atom is so overloaded that it has begun to cast off pieces of itself, as a deluded man might tear off his clothes. In a frenzy to achieve a state of rest, it slings off a missile of two protons and two neutrons at a velocity fast enough to whip around the circumference of the earth in roughly two seconds."

4. IF YOU INGEST IT, THANK YOUR KIDNEYS FOR KEEPING YOU ALIVE.

Traces of uranium appear in rock, soil, and water, and can be ingested in root vegetables and seafood. Kidneys take the burden of removing it from the bloodstream, and at high enough levels, that process can damage cells, according to the Argonne National Laboratory. But here's the good news: After short-term, low-level exposures, kidneys can repair themselves.

5. URANIUM MADE FIESTA WARE COLORFUL … AND RADIOACTIVE.

Before we recognized uranium's potential for energy—and bombs—most of its uses revolved around color. Photographers washed platinotype prints in uranium salts to tone otherwise black and white images reddish-brown. Added to glass, uranium gave beads and goblets a canary hue. Perhaps most disconcertingly, uranium makes Fiesta Ware's red-orange glaze—a.k.a. "radioactive red"—as hot as it looks; plates made before 1973 still send Geiger counters into a frenzy.

6. "TICKLING THE DRAGON'S TAIL" WAS KEY TO MAKING THE FIRST ATOMIC BOMBS.

Uranium occurs naturally in three isotopes (forms with different masses): 234, 235, and 238. Only uranium-235—which constitutes a mere 0.72 percent of an average uranium ore sample—can trigger a nuclear chain reaction. In that process, a neutron bombards a uranium nucleus, causing it to split, shedding neutrons that go on to divide more nuclei.

In the 1940s, a team of scientists began experimenting in the then-secret city of Los Alamos, New Mexico, with how to harness that power. They called it "tickling the dragon's tail." The uranium bomb their work built, Little Boy, detonated over the Japanese city of Hiroshima on August 6, 1945. Estimates vary, but the detonation is thought to have killed 70,000 people in the initial blast and at least another 130,000 more from radiation poisoning over the following five years.

The same property that powered bombs is what now makes uranium useful for electricity. "It's very energy dense, so the amount of energy you can get out of one gram of uranium is exponentially more than you can get out of a gram of coal or a gram of oil," Denise Lee, research and development staff member at Oak Ridge National Laboratory, tells Mental Floss. A uranium fuel pellet the size of a fingertip boasts the same energy potential as 17,000 cubic feet of natural gas, 1780 pounds of coal, or 149 gallons of oil, according to the Nuclear Energy Institute, an industry group.

7. THE EARTH CREATED ITS OWN NATURAL NUCLEAR REACTORS BILLIONS OF YEARS AGO.

In the 1970s, ore samples from a mine in what is now Gabon came up short on uranium-235, finding it at 0.717 percent instead of the expected 0.72 percent. In part of the mine, about 200 kilograms were mysteriously absent—enough to have fueled a half-dozen nuclear bombs. At the time, the possibility of nuclear fission reactors spontaneously occurring was just a theory. The conditions for it required a certain deposit size, a higher concentration of uranium-235, and a surrounding environment that encouraged nuclei to continue splitting. Based on uranium-235's half-life, researchers determined that about 2 billion years ago, uranium occurred as about 3 percent of the ore. It was enough to set off nuclear fission reactions in at least 16 places, which flickered on and off for hundreds of thousands of years. As impressive as that sounds, the average output was likely less than 100 kilowatts—enough to run a few dozen toasters, as physicist Alex Meshik explained in Scientific American.

8. AS A POWER SOURCE, IT'S "PRACTICALLY INFINITE."

A 2010 study from MIT found the world had enough uranium reserves to supply power for decades to come. At present, all commercial nuclear power plants use at least some uranium, though plutonium is in the mix as well. One run through the reactors consumes only about 3 percent of the enriched uranium. "If you could reprocess it multiple times, it can be practically infinite," Stephanie Bruffey, a research and development staff member for Oak Ridge National Laboratory, tells Mental Floss. Tons of depleted uranium or its radioactive waste byproducts sit on concrete platforms at nuclear power plants and in vaults at historic weapons facilities around the country; these once temporary storage systems have become a permanent home.