Abstract: An improved drilling fluid for use in a drilling operation. A drilling rig has a drill string for forming a borehole while drilling fluid flows down the drill string to a drill bit and back up the borehole annulus to a mud pit. The improved drilling fluid of this method enables the borehole to be extended through a geothermal formation having a temperature up to 500 degrees F. The drilling fluid has a highly viscous thixotropic fluid added thereto during a drilling operation in order to achieve a mixture having at least 1/2% polymerized alkali metal silicate that is formed by combining an aqueous acid solution with an aqueous alkali metal silicate solution, whereby a polymerized alkali metal silicate gel is produced followed by shearing of the gel to impart thixotropic properties thereto.