Abstract: A Cartesian diving toy having a transparent vessel for supporting a body of liquid. The vessel has an opening spanned by a flexible diaphragm for selectively applying increasing and decreasing pressure on the liquid in the vessel. A diving assembly is immersed in the liquid and is movable up and down within the liquid in response to changes in the pressure on the liquid. The diving assembly is in the form of a simulated hot-air balloon having a hollow shell for holding a quantity of gas. The shell has an opening at the bottom thereof through which a flexible straw is inserted for supplying the gas. A simulated gondola depends frm the bottom of the shell. A weight is provided within the shell for maintaining the balloon erect while immersed within the liquid.