GPS spoofing has emerged as a serious threat to maritime security, yet its global prevalence, persistence, and structure remain largely unmeasured. In this paper, we present the first large-scale measurement study of maritime GPS spoofing, using global Automatic Identification System (AIS) data, which contain the GPS coordinates broadcasted over time by ships across the world. We focus on large-scale regional spoofing, where external interference displaces many vessels across an area at once, leaving a recognizable signature of physically implausible motion correlated across ships; our motion-aware, marine-specific framework identifies this signature and grades the evidence for GPS spoofing in each region it finds. Applying our approach to AIS data from over 367,000 vessels collected between late November 2024 and early February 2025, we identify 31 persistent anomalous hotspots across high-traffic maritime regions, at least 22 of which show strong evidence of GPS spoofing, with spatial and temporal structure aligning with regional conflict and economic sanctions. Notably, our method found that the spoofing activity in the Red Sea responsible for the highly-publicized grounding of the 75,000-ton container ship, MSC Antonia, was ongoing months before the incident, which has not been previously documented. Similarly, we detected persistent spoofing in the Strait of Hormuz over a year before the 2026 Iran war brought commercial shipping through the Strait to near-standstill. Together, this work establishes GPS spoofing as a widespread, recurring, and measurable threat to global maritime navigation.

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