The purpose of this research was to provide further insights on bubble-induced agitation of heated bulk liquid. Fundamental studies on the bubble disturbance of a stratified thermal layer were carried out for a 6 mm sphere-volume equivalent diameter air bubble suspended in water (Eo=1.2). A video digital image system and thermochromic liquid crystals were used to reproduce the bubble movement as well as the wake drift of the liquid. A three-dimensional interface tracking simulation was used as a numerical tool. The results have revealed a long open wake region that is formed along the fluctuating bubble path. The amplitude of the bubble wake is much larger than that of the bubble path. In addition to longitudinal mixing, strong lateral mixing is also caused by the movement of vortices in the transverse direction. Details of numerical simulations have revealed a wake that tends to form a chain of Omega shaped vortex rings. These "rings" are connected to each other in the near wake region. The coherent effects further downstream lead to more complex vortex patterns in the far wake. The disintegration of the coherent chain of vortices due to bubble surface deformability is highly complex and not clear at this stage. A comparison with liquid crystal temperature response has revealed that the time scale of the mixing is much longer than the ascending bubble residence, approximately 8 s as obtained by several experimental runs. A transverse propagation of entrained cold water has given an estimate of the bubble-induced diffusion to be about 170 times larger than the diffusivity of momentum. The bulk liquid agitation cannot be correctly modeled without taking into account the bubble size dependent wake structure. (C) 2011 Elsevier Ltd. All rights reserved.