  The ferromagnetic material in a shipwreck on the seabed causes a modification
to the earth's magnetic field which can be measured at the surface. Proton
magnetometer measurements at the surface are used to locate wrecks. Here I
discuss how to interpret such data to explore the shape and orientation of the
shipwreck on the seabed. I give details of how to model shipwrecks and deduce
the magnetic signal that results. I also discuss how to analyse data in a more
general way. As examples, I present and analyse data on the shipwrecks of
YSTROOM and BOUBOULINA (ex COLONEL LAMB).
