Belgian research institutes have played an important role in the evaluation of deep geological repositories as a viable option for storing spent nuclear fuel. Working with Euratom funding, the Fibre Optic Sensing Research Centre in Belgium attempted to transfer expertise in optical fibre telecommunications applications to repository monitoring. The work took place at the HADES underground research laboratory, which was constructed by the Belgian Nuclear Research Centre. The idea behind the SOMOS project was to develop optical fibre-based temperature and radiation sensors and distribute them to a repository via capillary tubes. Stainless steel tubes installed at HADES during the CORALUS project were used to test the feasibility of the SOMOS concept. The geometry of the CORALUS tubes proved extremely challenging, but temperature sensors based on carbon-reinforced Tower grating fibres were successfully deployed. However, further research is necessary in order to reach the same milestone for the more fragile radiation sensors. The Fibre Optic Sensing Research Centre and its SOMOS partners are following up on these positive results.