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It has long been recognized that New Orleans is subsiding and is therefore susceptible to catastrophic flooding. Here we present a new subsidence map for the city, generated from space-based synthetic-aperture radar measurements, which reveals that parts of New Orleans underwent rapid subsidence in the three years before Hurricane Katrina struck in August 2005. One such area is next to the Mississippi River–Gulf Outlet (MRGO) canal, where levees failed during the peak storm surge: the map indicates that this weakness could be explained by subsidence of a metre or more since their construction. To make the subsidence map, we used 33 scenes recorded from Canada’s RADARSAT satellite. The technique involves interferometric phase comparison of 33 synthetic-aperture radar images taken at different times along the same nominal orbit, and exploits points on the ground that strongly reflect radar, termed ‘permanent scatterers’ (for details of the analyses, see supplementary information). Figure 1 shows the average distance (range) changes of the permanent scatterers along the direction of radar illumination over the three years (2002–05) during which data were collected. It indicates that subsidence was widespread during this period. All results presented here are range changes in the direction of radar illumination, but it is reasonable to assume that the principal motion was vertical. The mean and standard deviation in rate of range change for all the point targets was 5.6 2.5 mm yr 1 (negative rates indicate subsidence). The maximum rate observed was 29 mm yr . If surface motion was purely vertical, the corresponding mean and maximum subsidence rates were 6.4 and 33 mm yr , respectively. As global sea level is rising at an average rate of about 2 mm yr 1 (ref. 1), most of New Orleans is subsiding relative to the global mean sea level at an average rate of about 8 mm yr . Our data have exceptional spatial resolution, allowing assessment of the motion of individual buildings and other structures. Comparison with high-resolution aerial photographs shows that many of the scatterers are located at the intersection of a road and the wall of a building, where the road is roughly parallel to the satellite flight path (perpendicular to the radar look direction), or on building roofs (see supplementary information). The mean elevation of scatterers is therefore considerably higher than the mean ground elevation (Fig. 2). Parks and other areas of vegetation have few or no permanent scatterers. Our subsidence rates represent a three-year average. Although subsidence may vary with time, the average range-change rate we obtain is very similar to recent estimates of subsidence from ground-based levelling, and we believe our estimates reflect rates over a longer term. The geography of subsidence also shows high rates in the areas of Lakeview (south shore of Lake Pontchartrain) and Kenner (near Louis Armstrong New Orleans International Airport), in east New Orleans and in regions bordering St Bernard Parish. Lakeview is augmented with engineered fill, whereas the other areas are former wetlands, now drained and urbanized. The wetlands have highly organic soils; their subsidence results from drainage projects that promote soil desiccation, oxidation and compaction. In east New Orleans, subsidence rates have historically been the highest in south Louisiana, producing the lowest topography (3 to 5 m below sea level) and deepest flooding — the flooding after Hurricane Katrina caused a significant number of deaths. Subsidence here may also be increased by motion on the active Michoud fault. Parts of St Bernard and Orleans Parishes west of Lake Borgne are experiencing subsidence rates of more than 20 mm yr , including the levee system along the MRGO canal (Fig. 1, insets). Parts of this levee system were breached BRIEF COMMUNICATIONS