The magma-filled channels within a volcano’s caldera can be vast. Today’s invention is a submarine that is capable of travelling within such a flow of magma (and making detailed pressure and temperature measurements).
This might be useful for vulcanologists or just adventure sports enthusiasts. In reality, it’s much more likely to be an unmanned drone.
To withstand the sub-surface temperature and pressure, such a machine would need to have the structure of a heavily-insulated, spherical vacuum flask, made of carbon steel.
Even with an evacuated layer and other insulation, the molten rock temperature of say 1500 celsius would allow only short-duration ‘dives’ until the interior temperature rose to dangerous levels.
Such voyages would have to take into account significant buoyancy and viscous forces, and could thus be propelled by a large, slow-revving screw mechanism.