And that's where ordinary sailors come in. To assist Richard and his colleagues, all you had to do was build your own Secchi disk, a device first developed in 1865. The disk, made from wood, plastic or metal, needs to be 30 mm wide and painted matte white. It requires enough weight attached to the bottom to sink and either a long surveyor's tape measure or 50-meter rope marked off in 20-centimeter intervals attached to its topside. To use it, once you're offshore, stop your boat, lower the disk until it disappears and then enter the data — depth, location, water temp (if available) and any notes — into the Secchi app on your phone. Once you're back home and connected to the internet, your data is uploaded and added to Seafarer's database. Simple, huh?