The surface temperature of Titan1 is about 95 K, between the melting point (90.6 K) and the boiling point (118 K) of methane at the total surface pressure of 1.6 bar. Voyager 1 IR observations1,2 suggest a methane abundance in the lower atmosphere >0.19 of saturation. Comparison with the mean H2O abundance in the Earth's atmosphere suggests1 a CH4 saturation level ≅0.5. Thus the volume mixing ratio of methane is probably about 6% and methane begins to condense out some 40 km above the surface. It is therefore a natural speculation that methane exists in the liquid state on the surface of Titan. The atmospheric CH4 corresponds to a liquid layer ~10 m deep. But it is unlikely that the buffer for atmospheric methane would have a capacity equal only to that of the atmosphere itself. On Earth, the quantity of water in the oceans is on average several times 104 the quantity of water vapour in the atmosphere. Thus, if Titan has oceans of liquid CH4, they are probably >100 m deep. We argue here that, if this ocean exists, the present high eccentricity of Titan requires its depth to be >400 m, and that such an ocean can be detected by radar techniques.