Pennsylvanian Sydney Coalfield, Nova Scotia, Canada, is submarine as coal mining took place beneath the North Atlantic Ocean. All collieries were shut down in 2001. The coal was on average sulfurous, pyrite-rich, (ca. 5% total sulfur), which on exposure to the atmosphere oxidized rapidly to form definite sequences of secondary hydrated-sulfate minerals. Thus, the collieries were veritable laboratories for studying in situ hydrated-sulfate paragenesis in roof rocks-coal-floor rocks. Based on these studies, and on direct observations over several years, a conceptual model of roof-rock instability is proposed. This model explains rock falls in tunnels (slopes) during once-operating coal mines (short-term hazard), as well as surface subsidence in land areas undermined long ago by room-and-pillar methods in the Sydney Coalfield (long-term hazard). The model invites testing in other Pennsylvanian coalfields that have similar geochemical conditions and employ similar mining methods. (c) 2005 Elsevier B.V. All rights reserved.