Although recent compromises and admissions have given new credibility to claimed encounters of Man-in-the-middle (MitM) attacks on SSL/TLS, very little proof exists in the public realm. In this paper, we report on the development and deployment of Crossbear, a tool to detect MitM attacks on SSL/TLS and localise their position in the network with a fair degree of confidence. MitM attacks are detected using a notary approach. For the localisation, we use a large number of traceroutes, conducted from so-called hunters from many positions on the Internet. Crossbear collects this data, orchestrates the hunting from a central point and provides the data for analysis. We outline the design of Crossbear and analyse the degree of effectivity that Crossbear achieves against attackers of different kinds and strengths. We also explain how analysis can make use of out-of-band sources like lookups of Autonomous Systems and geo-IP-mapping. Crossbear is already available, and 150 hunters have been deployed on the global PlanetLab testbed.