We begin with a brief review of what you should be able to do based on the material presented in lectures 4-5. This lecture focuses on measurement of gravity at sea. The full range of approaches is summarised here. The first gravity measurements at sea were made from submarines (more stable than ships) – the method is described here and here. Ocean floor gravimeters provide the most stable measurements – one such instrument is shown here. Shipboard gravimeters are designed to minimise and measure the large accelerations experienced by a ship – there are two main approaches to doing this, but the most common is the use of a stabilised platform; such a platform is illustrated here. Examples of shipboard gravimeters include the La-Coste Romberg, Graf-Askania, Bodenseewerk, vibrating string, and Bell gravimeter. The Graf-Askania and Bodenseewerk gravimeters are illustrated here, and the Bell gravimeter here. A recent development is the measurment of gravity from submersibles. Initially the submersible had to be stationary for each measurement, leading to a sparse dataset as illustrated here, but more recently measurements have been taken underway. Such measurements can resolve small features such as individual dykes, and allow the application of Nettleton’s method to estimate the density of the seafloor.