If you want to understand what happens when people stop having kids while life expectancy keeps rising, look no further than Japan.

The country's population is falling faster than ever before, and it's prompted economists to call the nation a "demographic time bomb" — defined as a place where fertility rates are falling at the same time that longevity is increasing.

Some experts have even created their own doomsday clock for when Japan will go extinct.

But Japan just happens to be the most extreme case of what's happening in a number of countries around the world. People are working longer hours, fertility rates are falling, and future economies are being put at risk.

Here are just some of the countries that could become demographic time bombs over the next 20 years.