The Megalith Highway: Did the Idea of the Great Stone Tomb Travel by Sea?

Megalithic tombs of strikingly similar design were built along the Atlantic edge of Europe, from Iberia to Orkney and Scandinavia, and one long-standing explanation is that the idea travelled by sea. I test that story with open data. From an OpenStreetMap extract I classify 13,280 megalithic monuments in 34 European countries into twelve types and set them three tests. First, 29.1% stand within 10 km of the sea, against 12.8% of random land in the same countries, but only 22.5% of non-megalithic barrows in the same country mix; a bootstrap over 50 km cells gives 22.2% to 37.3% for the megaliths, so the margin over barrows is not secure. Second, I link 254 monuments to radiocarbon dates in the p3k14c database by documented name rules, calibrate with IntCal20 and compare regional start dates. North-west France has the earliest start (10th percentile 4885 BC, 95% interval 5145 to 4382 BC), and tombs in Atlantic-facing regions begin earlier than those elsewhere (median 3465 against 2590 BC), but within regions a tomb's start is unrelated to its distance from the sea. Third, 47% of megaliths lie within 50 km of the Atlantic and northern seas, against 27% of random land, with long barrows, stone rows and stone circles most strongly Atlantic; tombs as a whole are no more Atlantic than barrows. The evidence is consistent with a maritime spread but cannot establish it, and coverage of both monuments and dates is uneven. Interactive version: https://tristan-hotham.github.io/megalith-highway/. Data and code: https://github.com/Tristan-Hotham/megalith-highway. Blog post: https://blog.tristanhotham.com/2026/10/27/megalith-highway/.