Traceroute is widely used to report the path packets take between two internet hosts, but the widespread deployment of load balancing routers breaks a basic assumption -- that there is only a single such path. We specify an adaptive, stochastic probing algorithm, the Multipath Detection Algorithm (MDA), to report all paths that probes can follow between a source and a destination. We establish the foundations of, and show how to calculate, rigorous statistical guarantees for the discovery of the entire multipath route. We explore algorithm cost/guarantee tradeoffs in real experiments and show the inadequacy of the classic practice of sending three probes per hop.