1. A method for navigating an unmanned undersea vehicle (UUV) equipped with a beam steered downward looking sonar, the method comprising: receiving a plurality of three-dimensional acoustic reflectivity reference data sets, each of the plurality of reference data sets being associated with a reference location; receiving sonar data and other sensed data, the sonar data resulting from at least one sub-bottom feature; accumulating the sonar data to produce a three-dimensional representation of the at least one sub-bottom feature; locating a match between the three-dimensional representation and one of the plurality of reference data sets, the match having a match location; and combining the match location with the other sensed data to navigate the UUV.