It's 250 years since Port-au-Prince was hit by an earthquake, but to geologists the rapidly growing city has long seemed vulnerable to this most terrifying of natural disasters.
The Haitian capital sits almost on top of a geological fault line that has been steadily stressed for two and a half centuries, and the rocks had to give at some point. The impossible question was when.
In this special edition of One Planet, the BBC's Roland Pease talks to the geologists who only last year were warning of Haiti's peril, and hears about the lessons not only for Port-au-Prince, but for cities around the world that could be levelled in a moment, with no warning.
Mike and the usual One Planet team are currently on a short break to prepare for the next series. But they'll be back the end of January. You can keep up to date with developments at One Planet HQ by joining in the conversation on Facebook - the link's below. Or alternatively, email the team at Oneplanet@bbc.com.