Abstract: Following the conventional mining of rich oil shale for conventional above ground retorting, by horizontally extensive room and pillar mining operations, additional shale oil and related product may be secondarily recovered from the oil shale formation by in-situ retort formation and combustion. The secondary recovery steps would involve initially the identification of a large horizontally extending area, normally located between barrier pillars, which may be blocked off for the in-situ retort. Suitable shale oil drain arrangements may then be provided along one lower edge of this in-situ retorting area. Along the opposite lower edge of the in-situ retorting area, conduits may be provided for withdrawing product gas from the retort area. The retort area may then be sealed, and rubblized by blasting the pillars and caving in the roof to provide the appropriate 15% to 30% void volume for in-situ retorting.