The flow of heat from the interior of a planet tells us about its structure, composition, and – most importantly – its thermal evolution. Despite the large number of planetary probes on the job already, these investigations are only beginning. Towards this end, DLR is developing the HP³ probe (heat flow and physical properties package). HP³ would be the first probe after Apollo to measure the heat flow in the subsoil of a terrestrial body. It comprises anelectro-mechanical 'mole' that will burrow several metres deep into the soil of Mars, pulling a flat cable with heat sensors after it. These heat sensors will measure the temperature profile and thermal conductivity of the soil, data from which heat flow can be determined. HP³ will be one of the experiments carried by the lander module of NASA's proposed InSight mission scheduled for launch in 2016.