Moody’s Investors Service downgraded its rating of China’s sovereign debt one notch on Wednesday, citing concerns over growing debt in the country, which has the world’s second-largest economy. In recent years, as China’s stunning economic performance of past decades has become difficult to sustain, the country has used debt to fuel growth.

Now, Moody’s says, China will have to borrow more and more to maintain the levels of economic growth the government wants. The concerns Moody’s raises will sound familiar to those who follow the Chinese economy closely. Expressed by one of the world’s top credit ratings agencies, however, the misgivings will be harder to ignore.

Why Is China’s Debt a Problem?

In short: China doesn’t just have debt; it has a lot, and has accumulated it very fast.