We present a comprehensive review of historical geomagnetic data collected over the last 2 decades by many workers, culminating in the largest such compilation in the world. It spans over 4 centuries, from 1510 to 1930 inclusive, and consists of 151,560 declinations, 19,525 inclinations, and 16,219 intensities. The greater part of the new data set comprises unpublished observations recorded by mariners engaged in merchant and naval shipping across the world's oceans. Earlier published compilations and printed accounts of voyages have also been included, in particular when their original sources were inaccessible, missing, or no longer extant. The current effort has brought together an unprecedented number of early observations of the Earth's magnetic field, in standardized format, of potential use in many areas of geophysical research.