Most people confuse Mt. Nemrut of Bitlis with that of Adiyaman where the giant heads of kings lay. This Nemrut is not that. This is a 3050 meters high quiete volcano at the edge of Lake Van south of Suphan Mountain. The cone Base is 350 Km square and the opening of the shute is about 48 km square. The Lake inside the crater is 13 km square with an average depth of 100 meters.
To be honest with you if you are not a Volcanologist, the Crater Lake of Nemrut mountain is one of those things that you can avoid in Van region. Granted it is a nice to drive hours in this natural wonder of a volcano that erupted 1440 BC the last time, but the travel from the rim of the volcano to its lake inside takes too much time. The road is dirt and not smooth. Also it is covered with snow eight moths of the year.
However, I would definitely recommend you to go to the upper rim of the volcano from Tatvan side. The road is asphalt and will not take much time from the Lake. There you will see perhaps the best view of Lake Van and the town of Tatvan in the south-east. And, just turn around towards west and you will see the huge crater of the Mount Nemrut with its small lake in the base of it. The view is beautiful from both sides. From there I would highly recommend to go back to the main road and head towards Ahlat… Enjoy…