This rectangular building is shown for the first time on the 1st edition map of 1899, as part of the Nant Hir mine and labelled as the engine house. It is stone-built, with coursed stone and lime mortar bonding, and measures 5.2 metres by 4.5 metres. The eastern end is inserted into the ground to a depth of at least 1 metre. The end walls are only 0.70 metres thick but the lateral walls are 1.5 metres thick, leaving the internal area 1.5 metres wide. In the north wall there are at least 3 metal pins, and externally the wall stands up to 2 metres high. On the west northwest side of the building is a levelled area defined by very low foundations, 6 metres by 5.25 metres. J.J. Hall, Trysor, 20 March 2012