Some of the most critical reactions for the understanding of the synthesis of elements in stars are the stellar neutron sources. These are reactions that produce neutrons during certain phases of stellar burning, which in turn initiate slow neutron capture reactions (s-process) gradually building up heavier elements towards lead and bismuth. The s-process depends sensitively on the neutron flux or the strength of the neutron sources. A measurement of these reactions in laboratory experiments is handicapped by the fairly large background from cosmogenic neutrons above ground. The accelerator facility CASPAR has been build 4950 ft underground to measure these neutron sources in a cosmogenic background free environment. The challenges and the status of this project will be discussed as well as the impact of the experiment on s-process nucleosynthesis.