This paper is concerned with the structure of the wake behind gas bubbles rising at high Reynolds numbers in highly purified water. It describes a schlieren optics technique to visualise the wake. The technique does not contaminate the water, and so does not affect the zero-stress condition at the bubble surface. It is first shown that zigzagging bubbles have a double-threaded wake of which the axially vorticity components periodically switch sign; some distance downstream of the bubble the wake is unstable. It is explained that this wake structure signifies that the bubble experiences a lift force; the magnitude of the lift force is estimated by two different indirect methods. The results suggest that the zigzag motion is not maintained by periodic vortex shedding, contrary to what was found in earlier investigations. In the second part of the paper we study the collision of bubbles with a vertical wall. It is shown that in the collision the bubble loses its wake, which subsequently impinges on the wall and reorganises into a coherent, approximately spherical, vortex blob. The presence of vorticity plays a crucial role in the collision. The experimental findings have been incorporated in a simple model to describe the path of a bubble after the collision, which is shown to yield good agreement with what is observed in the experiments.