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Mobile underwater sensor networks are usually featured with three-dimensional topology, high node mobility and long propagation delays. For such networks, geographic routing has been shown to be very suitable. However, routing voids pose great challenges on the greedy policy used in most geographic routing protocols. The problem is more severe for underwater sensor networks because of node mobility and three-dimensional topology. In this paper, we propose a void avoidance protocol, called Vector-Based Void Avoidance (VBVA), to address the routing void problem in mobile underwater sensor networks. VBVA adopts two mechanisms, vector-shift and back-pressure, to handle voids. Vector-shift mechanism is used to route data packets along the boundary of a void. Back-pressure mechanism routes data packets backward to bypass a concave void. VBVA handles the routing void problem on …