The Extension Prospect mine is situated at Extension on the south end of Harewood Ridge. The mine operated along a faulted section of the Wellington seam which occurs within the Early Campanian Northfield Member of the Upper Cretaceous Extension Formation, Nanaimo Group. Refer to the Bebans mine (092GSW026) for a description of the Wellington Seam and the Nanaimo Coalfield operations.

The slope was originally opened by the old Vancouver Coal Company in 1899 and driven for a distance of about 198 metres from the surface. A counter-slope was also driven a distance of 46 metres and connected to the main slope by a small shaft for ventilating purposes. In August of 1940, Canadian Collieries (Dunsmuir), Limited, dewatered the slope to a depth of 168 metres and turned off levels to the right and left to prove the seam. Two new levels were started at a point 46 metres from the portal and advanced in coal varying in thickness from 1.2 to 2.1 metres.

Highly volatile, bituminous rank coal was mined from 1941 to 1947 and approximately 17,433 tonnes of coal were produced before the mine was abandoned on July 18, 1947.