Mexico City is sinking. The overexploitation of subterranean water reserves has landed the city - which on Thursday hosts the World Water Forum, a ministerial conference aimed at raising consciousness about water as a valuable commodity - with a battle against subsidence and an increase in flooding.

Santiago Cortéz knows all about the problem. In 1985 he and his wife began building their house in Molinos, a poor neighbourhood on the south-eastern outskirts of the city. The modest two-storey construction had three large steps leading up to the front door.