Recent observational results show that water is prevalent throughout our solar system, including in many previously unexpected locations, such as in the main asteroid belt. Water is essential for life as we know it and an important tracer of the formation and evolution processes in our solar system, especially when preserved in minor bodies. The ‘Main Belt Comets’ (MBCs) are a recently identified population with comet-like morphology and asteroid-like orbits, which appear to contain buried water ice dating from the formation of the Solar System. With activity levels orders-of-magnitude lower than typical comets at perihelion, MBCs present an observational challenge, but also an opportunity to better understand the physics of mass-losing bodies in the low-activity regime that is rarely explored – and one that the early phase of the Rosetta mission showed to be more complex than expected. Our ISSI international team brings together experts in comet observation, including those with expertise in dust tails and in detecting water across different wavelength ranges, and modellers who work on understanding active bodies, from thermal models of the survival and sublimation of water ice, to the release and flow of dust grains. The question of how to observe the very weak gas release from MBCs, and therefore conclusively show that they contain water, is one key topic to be addressed.