While evidence suggests that the dark areas in SAR imaging of Titan's polar regions are in fact lakes filled with liquid methane and ethane, one crucial measurement was still missing. Now, we finally have radar altimetry data for a track that crosses Ontario Lacus, a 235km dark feature near Titan's south pole:
http://www.lpi.usra.edu/meetings/lpsc2009/pdf/1990.pdf
The altimetry profile shows that Ontario Lacus is very flat and reflective--both consistent with a deep body of liquid hydrocarbons. The lake sits in a depression that is 300 meters deep.
--Chris