A method for sealing boreholes in the earth and particularly around well casings extending into boreholes in the earth comprises the placement of a plurality of discrete, liquid-absorbing annular sealing elements or disks formed of relatively dry, solid, particulate or granular liquid-swellable clay material, such as bentonite, or bentonite clay which swells in size upon contact with and absorption of liquid encountered in the borehole, The elements are characterized by a disk-like planar shape having parallel, flat opposing faces and a central hole slightly larger in diameter than the cross-section of a well casing when present in a borehole. The disks are placed successively on the well casing and move down into the borehole until a stack of disks of the desired height is formed to substantially fill the space or void between the well casing and the borehole wall. When liquid comes in contact with the disk in a stack, the particulate material of the disks begins to swell and enlarge to complletely fill the borehole or the space between the well casing and the borehole wall to provide a liquid-tight seal around the well casing within the borehole.