From the first navigations of the Southern Ocean in the 16th century, lands which subsequently proved to be nonexistent had from time to time been reported in these waters. Polar historian Robert Headland of the Scott Polar Research Institute has suggested various reasons for these false sightings, ranging from "too much rum" to deliberate hoaxes designed to lure rival ships away from good sealing grounds. Some may have been of large ice masses that were carrying rocks and other glacial debris—dirty ice can appear convincingly similar to land. It is also possible that some of these lands existed, but later became submerged after volcanic eruptions. Other sightings may have been of actual land, wrongly located through observational errors due to chronometer failure, bad weather or simple incompetence.