The industrial revolution spurred an urbanization movement as new factories created jobs and economic growth that drew workers from rural areas and other countries. For instance, in 1850 New York City had 590,000 residents and by 1900 there were 3.8 million New Yorkers. Source. Likewise, London’s population grew from 2.6 million to 6.5 million during that same period. Source.

The booming growth of cities generated a host of issues including air pollution, lack of trash disposal and sanitation, and rampant disease from having so many people packed together. Factory laborers, many of them immigrants, commonly lived horrible conditions in slum tenements that were crowded and dark. (I’d highly recommend visiting New York’s Tenement Museum for insight into those harsh living conditions.)

Transportation of people and freight was primarily powered by horses until the early 1900s. People traveled in carriages, buses, and streetcars on rails — all drawn by horses. Goods and materials moved around cities by horse-drawn wagons. So as a city’s human population boomed, so did its horse population. Around the turn of the century, an estimated 170,000 – 200,000 horses were living in New York City. Source.

All those horses generated a lot of waste as a single horse produces between 20 – 30 pounds of manure and about two liters of urine a day. That means that New York’s 200,000 horses generated about 5 million pounds of manure a day along with about 100,000 gallons of urine. And feeding all those horses was problematic as an average horse eats about 10-20 pounds of hay per day. Large swaths of farmland around major cities were devoted to producing hay and oats for horse feed. As the number of horses needed for transportation increased, so did the number of horses needed to transport horse feed into the cities and move manure out of the cities, creating a circular problem.

Disposal efforts couldn’t keep up with the generation of manure. Streets in major cities were caked in manure and “when it rained, the streets turned to muck. And when it was dry, wind whipped up the manure dust and choked the citizenry.” Source. In 1894, The Times of London predicted, “In 50 years, every street in London will be buried under nine feet of manure.” Similarly, one New York commentator “estimated that, unabated, horse droppings would rise to the level of third-storey Manhattan windows by 1930.” Source.

The smell and overall grossness weren’t even the biggest problems; the massive amounts of manure and urine attracted flies which spread disease, creating a major health crisis.

The manure problem seemed intractable. “When the world’s first international urban-planning conference was held, in 1898, it was dominated by discussion of the manure situation. Unable to agree upon any solutions—or to imagine cities without horses—the delegates broke up the meeting, which had been scheduled to last a week and a half, after just three days.” Source. The horse manure crisis was the biggest threat to continued urban growth and had no obvious solution.

But then in the early 1900s, the manure problem was solved not by discovering better methods of cleaning up after horses but by replacing horses as a means of transportation. By 1912 there were more automobiles in New York than horses, in 1917 the last horse-drawn streetcar was retired, and by 1930 — the year that three stories of manure were predicted — horses had been completely replaced.

The phrase “The Great Manure Crisis of 1894” is now used as a shorthand reference to how seemingly insurmountable problems can be rendered moot by technological advancements — even if the primary purpose of the technology wasn’t to solve that particular problem. As Stein’s Law states: “If something cannot go on forever, it will stop.”