Mini Sub detects the presence of something under Lake Ontario that baffled the Army and scientists alike

What Do You Mean?

The first time the team was told that they would be searching the depths of Lake Ontario they thought that they might get to know what they would be searching for, but there was nothing except the fact that in the year 1950 some local people saw something at the lake which just vanished into thin air in a blink of an eye. What did the local people see?