'The Flash' and 'Arrow' take place in Comic Book Toobworld. But even there, real world events have occurred as well.
Hawkman, also known as Carter Hall and as Khufu, mentioned that Vandal Savage murdered hundreds of thousands of people in China by causing the Huan He flood in 1887.
It was a tossed-off remark, but the audience should know more about the tragedy so that it isn't fully trivialized.
For centuries, the farmers living near the Yellow River had built dikes to contain the waters, which over time flowed higher because silt accumulated on the riverbed. In 1887, this rising river, swollen by days of heavy rain, overcame the dikes on around 28 September, causing a massive flood. Since there is no international unit to measure a flood's strength it is usually classified by the extent of the damage done, depth of water left and number of casualties.
The waters of the Yellow River are generally thought to have broken through the dikes in Huayuankou, near the city of Zhengzhou in Henan province. Owing to the low-lying plains near the area, the flood spread very quickly throughout Northern China, covering an estimated 50,000 square miles (130,000 km2), swamping agricultural settlements and commercial centers. After the flood, two million were left homeless. The resulting pandemic and lack of basic essentials claimed as many lives as those lost directly to the flood. It was one of the worst floods in history, though the later 1931 Yellow River flood may have killed as many as four million.