Imagine if the Chicago fire of 1871 had gone on for six months before anyone noticed. Imagine if the Johnstown flood in 1889 or the 1906 San Francisco earthquake had lasted six months, a year, two years, before anyone paid attention to it.
Building with wood, building on fault lines, building on floodplains, each era creates its own «natural» disasters.
Imagine a flood of dark green in the downtown of any major city, the office and condo towers submerged inch by inch.