The interaction between a laminar boundary layer and an open cavity is investigated experimentally for Reynolds numbers between 860 and 32300. Flow visualizations are carried out for different observation directions in order to understand the spatial development of dynamical structures. The study is conducted by changing the cavity length and height and the external flow velocity, and therefore the flow patterns inside the cavity. The issue is to emphasize the three-dimensional development of the flow. In particular, we show that the cavity dynamical structures are not due to secondary shear layer instabilities. Görtler-type vortices are developing inside the cavity for some parameters and their properties are discussed. The existence diagram of these instabilities is also provided.