Titan, the largest moon of Saturn, holds a global ocean underneath its frozen surface. This is a characteristic shared with multiple icy satellites in the outer solar system. In this talk, I will summarise the observations carried out to probe the internal structure of this body and in particular to reveal the presence of the liquid layer. They cover several fields of physics (thermodynamics, electrodynamics, geophysics, and classical mechanics) and most of them have been made possible in the last decade thanks to the Cassini-Huygens space mission. Then I will develop the classical mechanics aspect of the problem showing how a subsurface ocean affects the rotation dynamics of a rigid body.