Japan’s soaring national debt, already more than twice the size of its economy, has reached a new milestone, surpassing 1 quadrillion yen.

Let the word quadrillion roll around in your brain for a moment or two, because it is not something you hear every day. Quadrillion. 1,000,000,000,000,000. Really.

A paltry million is the numeral one followed by six zeros. A billion? Nine zeros. A trillion is getting up there: 12 zeros. But the mighty quadrillion has 15 of them.

The mind boggles. (Although it doesn’t googol: That one is followed by 100 zeros. And that’s the actual spelling. You can Google it.)

A quadrillion is a million billion, putting it into the kind of language used by middle schoolers to describe really humongous sums, along with gazillion and bazillion.

Measuring any currency in quadrillions brings to mind the hyperinflation of Germany between the wars, or Zimbabwe in the last decade. But a country with a real currency?

It is such a big and unusual word, describing such a big and unusual number, that its use is inconsistent: Bloomberg News used quadrillion in the headline of an early story Friday about Japan’s debt, but later in the day the stories and headlines referred to a "thousand trillion," which is not nearly as much fun.

Questions emailed to the Bloomberg editors responsible for those stories were not returned, suggesting perhaps a lexicographical quadrilliongate.

How much is a quadrillion? The entire human body is said to have just 100 trillion cells; it takes 10 of us to make a quadrillion. Jeff Bezos has a personal fortune of some $25 billion, allowing him to plunk down $250 million for The Washington Post, which is essentially how much money he might find by looking behind his sofa cushions. To get to a quadrillion dollars, however, we would have to have 40,000 Bezoses, or as many people as live in Prescott, Ariz. Neil deGrasse Tyson, the astrophysicist and director of the Hayden Planetarium at the American Museum of Natural History, helpfully offered a few other ways to think about a quadrillion.

"It would take you 31 million years to count to a quadrillion – one number per second, never sleeping," he said in an email, adding that "a quadrillion yen, stacked in 1,000-yen notes, would ascend 70,000 miles high."

He also wrote, although it is not clear how he would know such a thing, that "the total number of all sounds and words ever uttered by all humans who have ever lived is about 100 quadrillion."

"This figure includes all congressional debates and filibusters," Tyson wrote.

Compared with Japan, the U.S. national debt is a mere $16 trillion or so. But if you convert that number into yen, it comes to about $1.5 quadrillion. So it’s good to have a currency that conserves its zeros. Of course, that also means the total U.S. debt is even larger than Japan’s (although not, it should be noted, as a percentage of gross domestic product).

Hmm. Let’s not talk about that.

© 2013 The New York Times Company