We produced new multi-temporal Digital Elevation Models (DEMs) of the April 2007 summit collapse at Piton de la Fournaise from previously unused aerial photographs. This dataset reveals the precise temporal evolution of the collapsed volume and caldera morphological changes during the event. It provides a unique opportunity to study caldera formation, one of the most hazardous natural phenomena, for which relatively little scientific and quantified information is available. During this rare example of observed caldera formation, the summit started to collapse four days after the onset of a high-volume eruption at an unusually low elevation (at 20:48 UTC on April 5 th). Our new data show that during the first 30 hours, collapse was relatively fast (840 m 3 s −1 average), and continued for at least the following 12 days, at a slower rate (46 m 3 s −1 average), which had not previously been reported. On April 19 th , the collapse reached 96 % of its final volume, while the remaining 4 % was probably attained by May 1 st (end of lava emission at the vent). New infrared 3D mapping of the caldera floor made a year after the event demonstrates that post-collapse hydrothermal activity in the caldera is closely associated with the main ring faults active during the collapse, which are now preferential paths for fluids to reach the surface.