The number of executions worldwide rose 15 percent in 2013, according to an Amnesty International report, with the United States up with Iran, Iraq and Saudi Arabia as the countries executing the most people.

But one country stood out from the rest. In fact, Amnesty argued, China appeared to have executed more people than the rest of the world put together in 2013.

You can see how absurdly huge that number is by looking at this chart put together by Amnesty (click it to see a larger image):

When you look at the graphic above, however, you probably notice that the end of the chart for China fades out. That's because Amnesty hasn't been able to fully estimate the true number of executions in China.

This isn't a new thing. Last year, Amnesty was forced to reach the same conclusion for 2012. In fact, the last time Amnesty was able to estimate the number of executions in China with relative accuracy was in 2008, when it's believed that at least 1,718 people were executed.

The problem is that China treats the number of executions like a state secret, so observers are forced to use media reports and other means in an attempt to come up with accurate figures. Other groups have attempted to estimate the number of Chinese executions and have come to some extremely high numbers: The Dui Hua Foundation estimated that 4,000 were executed during 2011, for example.

Why is China's rate of execution so high? Well, for one thing, it's a huge country, and even at higher estimates, its per-capita rate of executions might be more comparable to small states that barely make a dent on the above chart. Another factor is the number of laws for which execution is a possible punishment in China – in 2011, it was reported to be 55, and included a number of economic crimes (for example, last year Zeng Chengjie, "China's Madoff," was executed in secret for fraud).

Despite the lack of transparency, China does appear to be trying to get the number of executions down – the 55 crimes punishable by death was a reduction of 13 from previous numbers, and some estimates have the number of executions halving over the past decade.