Lake Motosu (本栖湖, Motosuko) is the most western of the five lakes. It was once in fact part of a much bigger lake that included Shojiko and Saiko to the east, but an eruption of Mount Fuji in 864 saw a huge volume of molten lava flow down the valley and cut this larger lake into the three separate lakes we see today.
Motosuko has no water inflow or outflow, and it is understood that the water seeping back and forth through the minerals and rocks below are part of the the reason the water is so transparent. It is known for its exception scenic beauty, and a 1945 photograph by Okada Koyo of the reflection of Mount Fuji in the blue waters appears on the back of the 1,000 yen note.
Unfortunately, the lake is very difficult to reach by public transport. Fuji Kyuko do operate a bus to the area but it is infrequent (once every two hours). See the company website for details.