Inversion of continuous GPS measurements from the Pacific Northwest show the 2003 Cascadia slow earthquake to be among the largest of ten transients recognized here. Twelve stations bracketing slow slip indicate transient slip propagated bi-directionally from initiation in the southern Puget basin, reaching 300 km along-strike over a period of seven weeks. This event produced, for the first time, resolvable vertical subsidence, and horizontal displacement reaching six mm in southern Washington State. Inverted for non-negative thrust slip, a maximum of 3.8 cm of slip is inferred, centered at 28 km depth near the sharp arch in the subducting Juan de Fuca plate. Nearly all slip lies shallower than 38 km. Inverted slip shows a total moment release equal to Mw = 6.6 and a high degree of spatial localization rather than near-uniform slip. This suggests rupture concentrated along asperities holds for slow earthquakes as well as conventional events.