We study the causal relationship between geographic connectedness and development using one of the earliest massive trade expansions: the first systematic crossing of open seas in the Mediterranean during the time of the Phoenicians. We construct a geography-based measure of connectedness along the shores of the sea. We relate connectedness to economic activity, which we measure using the presence of archaeological sites. We find an association between better-connected locations and archaeological sites during the Iron Age, at a time when sailors began to cross open water routinely on a large scale. We corroborate these findings at the world level.