The first nadir-pointed radar altimeter observations of a known extraterrestrial lake were acquired by Cassini over Ontario Lacus during the T49 Titan pass on December 21, 2008. These data show the lake to lie in a 300m topographic depression and to be exceptionally flat. The altimeter echoes over the lake surface are narrowly-peaked and brilliant in intensity, consistent with a very smooth surface. Coincident passive radiometry indicates the lake to be highly emissive, consistent with a deep body of hydrocarbon liquids. Introduction : Near-IR images from Cassini ISS detected a 235-km long lake-shaped dark feature, later named Ontario Lacus, at ~183W, 72S [1]. Synthetic Aperture Radar (SAR) observations have mapped hundreds of similar hydrologic features in the north polar regions [2,3] and showed them to be both smooth and emissive, consistent with liquid hydrocarbons. Initial results from the southern hemisphere show a relative paucity of such features [4], an asymmetry which is not yet fully understood. Spectroscopic observations acquired by Cassini/VIMS in December 2007 determined [5] that at least a thin layer of liquid containing ethane was present at Ontario Lacus. Although close approaches on Radar flybys are usually for SAR imaging, T49 had a groundtrack directly across Ontario Lacus (Fig.1), providing our first opportunity to obtain nadir-pointed altimetry from ~1700km over a known or suspected lake. Topographic Results : A preliminary estimate of surface height using a predicted ephemeris is determined from the centroid of the altimeter echo waveforms (other approaches will be applied in future analyses). An improved ephemeris available some weeks hence will refine these absolute altitudes, yet it is already clear (Fig. 2) that Ontario lies in a broad depression some ~300m below its surrounds. This is consistent with the ~200-300m depressions interpreted as empty lakes in the northern polar regions [3] and indications that many liquid-filled northern lakes lie in steep-sided depressions [3]. Figure 1. Approximate groundtrack and representative width of the beam-limited altimeter footprint. Improved navigation may displace the beamtrack by roughly its width. Note that the background ISS image was acquired over 3 years before the T49 radar data.. 0 50 100 150 200 250 300 350 400 450 -1200 -1100 -1000 -900 -800 -700 -600 Along Track Distance (km) H ei gh t ( m ) -1000 -500 0 0 5 10 15 x 10 6