The accelerating impact of the climate crisis is forcing us to limit new construction. Rehabilitation is thus soon becoming a major mode of housing production. But how do we deal with buildings deemed obsolete, or hopeless situations? Some architects decided to take up this challenge: setting up a vacation home in a bunker, laying out a public library in a disused quarry, contriving a refuge in a collapsed manor... Instead of giving up, they identify and exploit opportunities afforded by the original structure. In so doing, causes presumed lost are reconceived as missed opportunities. A lesson in architectural optimism!