So, just for some additional context here: 20-30 million is a sort of "best guess" estimate. Nobody really knows how many Chinese died in the Taiping Civil War (really a more proper name than a rebellion, in my opinion, since it ranged across pretty much the entirety of China and featured several major factions essentially vying for control of the country). Some reliable sources such as Cao Shuji have ranges as high as 70 or even 100 million people killed. As there aren't really reliable census studies of the Chinese populace from this period, we don't know exactly how many people died, but 20-30 million is the accepted best guess by most Western historians. Ho Pingto, a modern historian of Chinese population numbers, however, feels this is a low estimate. Either way, pretty staggering.

And then, a note about why the death toll was so high: it was, as is the case with many human conflicts, not directly due to fighting per se, but more to disease and famine caused or exacerbated as a consequence of the war. Significant infighting among the Taiping factions, for example, hamstrung their economy and logistics and left some areas under their control unable to properly feed their populations.

To be sure, there were some very bloody battles. At the Third Battle of Nanking, which was the last major battle of the war, in which there were probably more than a million soldiers present, the Taipings lost more than 200,000 troops, and the victorious Qing army then set about looting and burning the city, causing many more civilian casualties.

As to why you haven't heard about it in history class... that I can't comment on with certainty. One reason might be that the Taiping Civil War overlapped in its final years with the American Civil War, something that Western, and certainly American, history classes are probably more likely to focus on. However, the Taiping Civil War was critically important in the history of China, and had many lasting effects -- not least of which was the continued perception of Christianity (and religion broadly) in China today.

/u/TheJucheisLoose and /u/DeSoulis have given you an answer about the Taiping Civil War and Hong Xiuquan, so let me tackle the other part of your question: "why haven't I heard about this absolutely devastating chapter of human history in history class?"

According to the irregular census we conduct here at AskHistorians, the vast, vast majority of our readers here are white, male, middle class or upper class, from the United States or another Western, English-speaking country, and in high school or undergraduate university.

Since our users are so young, I've no doubt that there's a lot of things they haven't heard about before. I bet you learn something new every day, right? As you get older, you encounter a lot of things for the first time, and it shouldn't be surprising that you haven't heard of the Taiping Civil War before. There's nothing wrong with that.

If you're a high school student in the United States, you've probably gone through a few years of history education in elementary and middle school. History isn't taught every year in every state, unfortunately, but you likely spent a year (probably in third or fourth grade) learning about your state or local history. That would have been followed in late elementary or early middle school by a course on U.S. history. Finally, at the end of middle school, you probably took a world history course.

Note that that outline will differ from state to state, since each state has different standards.

In high school, if you're interested in history, you might take one or two classes per year, alternating some form of U.S. and world history.

That's not a lot of time to learn. History is vast; it encompasses all of human endeavor, all of the billions of people living now and those who have ever lived. It's impossible to know it all ─ the best we can do is learn a fraction.

With that in mind, grade school and high school history programs aren't supposed to let you delve into a particular topic or subject. They're supposed to give you a basic set of tools with which to comprehend the world around you. These tools will give you what you need to pursue your studies further, if you're interested.

For that reason, American grade schools and high schools will focus on the kinds of history that are most likely to be critically important to you. They'll teach you a narrative about the United States and Western culture, which you will encounter every day. You'll likely be taught about the civilizations and nations that contributed to the rise of Western culture and how that culture led to the creation of the United States, and how the United States came to be what it is today. If you're at a smart program, you'll learn historiography, that history isn't a narrative ─ it's an argument.

In either case, your history courses will likely focus on the United States, on English-speaking nations, and what led to the creation of those English-speaking nations. Teaching that way isn't wrong or bad ─ it's just designed to give the widest population the lowest-common-denominator of historical education needed to cope in society.

For your average American high school student, the Taiping Civil War doesn't fit in. Because history is so vast, grade school and world history courses must zoom out. Even titanic events are small at this scale. The life's work of some of the most renowned people who ever lived will fit into a few paragraphs or a page. The Taiping Civil War is reduced to 10 seconds in a Ken Burns documentary you watched while the teacher was out sick.

Are events like Taiping neglected? Sure. But that's intentional.

If you're interested in a particular topic or subject, it's up to you to learn more about it. More importantly, it's up to you to learn what you don't know.

There are plenty of things I don't know. But I know that I don't know them. I might not know what they are, but I know there's more to the story, and so I keep looking. High school and college classes aren't designed to teach you everything. You have to go out and keep looking.