lean stratum with solvent raising the roof of the cavities by incrementally raising the insulating fluid level and allowing the cavities to grow laterally until the cavities connect application_number US 6322079 A description in accordance with the present invention a chimney is developed at the bottom of a borehole to aid in early connection of two or more developing cavities reference is now made to fig1 where two adjacent wells between about 60 meters and 130 meters apart are drilled down through overburden 17 and through potassium chloride rich stratum 16 into potassium chloride lean sodium chloride rich stratum 15 and to the bottom of what is shown as chimneys 7 and 8 the depth of the ultimate cavity shown in fig3 into stratum 15 is determined by 1 the minefield brine balance in terms of how much potassium chloride lean brine can be tolerated and maximizing the potassium chloride rich brine produced 2 the desirability to quickly develop the cavity and hence to develop in a quickly dissolving sodium chloride stratum and 3 the desirability to quickly produce potassium chloride to obtain a quick return on investment in drilling and cavity development costs hence one skilled in the art can determine from the above the optimum cavity depth into stratum 15 the well bores are cased with casing 1 and 2 to the depth at which the top of the chimneys 7 and 8 are to be established again those skilled in the art of solution mining can determine what this depth should be which determines the height of the chimney from the bottom of the.