Cavern formation is a widely reported phenomenon in agitated vessels processing liquids with a yield stress. This study employs electrical resistance tomography and flow visualization techniques to monitor the spread of a tracer in a vessel containing complex non-Newtonian media agitated using submerged recirculating liquid jets. The experimental media are aqueous solutions of xanthan gum with varying concentrations. It has been demonstrated that when agitated using submerged jet, the tracer is localized within a fixed volume of the vessel and does not spread with time in the more concentrated xanthan gum solution, a phenomenon that is analogous to a cavern structure found in mechanical mixing. A Herschel-Bulkely model with a positive yield stress can be fitted to part of the rheogram of the more concentrated xanthan gum solution. The less-concentrated solution cannot be fitted with a positive yield stress model, and when agitated in a vessel, the tracer front pervades the entire volume of the vessel and no cavern is seen, in experiments when all other conditions are kept equal.