 Complete the following phrase:  In ancient Roman and Greek times, there were instances of men swimming or diving for combat, sometimes using a hollow plant stem or a long bone as a snorkel. Diving with snorkel is mentioned by Aristotle 4th century BC. The earliest descriptions of frogmen in war are found in Thucydides History of the Peloponnesian War. The first instance was in 425 BC, when the 