Abstract: A method and system are provided for horizontal borehole mining a seam of material, in which a substantially horizontal borehole is provided through a seam of material, a retractable casing is disposed within the seam, and a high pressure drilling tool is disposed within said casing for excavation of the seam material to form an excavation cavity, the high pressure drilling tool having a high pressure jet, the direction of the high pressure jet being adjustable during the mining operation. As mining progresses the casing and high pressure drilling tool are retracted, so that the excavation cavity is extended along the length of the borehole back toward the origin of the borehole. The high pressure drilling tool can be either hydraulic or pneumatic.