A system for in situ retorting and recovery of carbonaceous values from underground formations of oil shale. A plurality of parallel access entries communicating with the surface extend horizontally at a level beneath the formation. Upright retort chambers are arranged at spaced intervals on opposite sides of each access entry. The bottom of each chamber is interconnected with one such access entry by means of a sealed transverse tunnel. The retort chambers are ignited from the top and the carbonaceous values released from the shale by a downwardly progressing heat front are collected at the bottom. Oil recovery conduits extend from the surface through vertically directed bore holes opening into the access entry, each such conduit branching at that point to interconnect with opposite pairs of retort chambers through the sealed tunnels. Further conduit means are provided for scavenging gaseous retort products through an annular space surrounding each of the oil recovery conduits so as to apply constant heat to the shale oil while it is being conducted to the surface. Means are also provided for insuring that a positive pressure is maintained in the access entries with respect to the bottom of the retort chambers.