This practice-based PhD critically investigates the aesthetic and spatial conditions that have turned the Mediterranean into a military-humanitarian border zone, dissecting the political anatomy of violence inflicted at and through the sea. It understands the maritime borders of the EU as a paradigmatic conflict zone in which new assemblages of power, legal arrangements and uneven patterns of mobility have emerged in relation to a vast, and yet patchy, surveillance apparatus. Contrary to the popular representation of the maritime territory as a homogeneous and empty expanse, the sea appears here as a technologically mediated space thick with events and complex relations between people, environments, and data. Recasting the notion of structural violence in aesthetic terms (i.e., as violence hidden in plain sight), this thesis further investigates documentary, humanitarian and cartographic practices that operate across this contested frontier and their role both in governmental practices of control and in migrants’ infrastructures of mobility. Part 1 (Genealogies) locates the current migration regime at sea within a longer genealogy of bordering technologies and aesthetic practices operating at sea. Part 2 (Liquid Traces) builds upon “Forensic Oceanography”, a project that I co-initiated in 2011 and which has mobilised geographic and media technologies (remote sensing, drift modelling, GIS, vessel tracking and others) to document the violence perpetrated against migrants in the Mediterranean. Here I read the maps, videos, visualisations and human right reports that I have co-produced during this project and that have been used as evidence in actual legal proceedings as attempts to challenge the regime of (in-)visibility imposed on this contested area. This thesis offers a new “cognitive mapping” of migration at sea by following my own situated encounters with the practices, policies, discourses and geographies that constitute the sea as a frontier