Horse dung was the crisis of all crises

In 1898, New York hosted the first International urban planning conference.  The agenda was dominated by horse manure, because cities around the world were experiencing the same crisis.  But no solution could be found.  “Stumped by the crisis,” writes Eric Morris, “the urban planning conference declared its work fruitless and broke up in three days instead of the scheduled ten.”  The world had seemingly reached the point where its largest cities could not survive without the horse but couldn’t survive it either.
And then the problem vanished.