Aliyas Lake known as Gapen Gath is a Big Tarn above the quaint little village Sissu (Lahaul). The lake is approx 2 kms long and is increasing continuously as the glacier feeding the lake is melting too quickly. Therefore, it has been marked as an unstable GLOF. However, few studies have now confirmed that the lake is stable and there is no chance for it to explode. Because of the fast glacier melting, the area is getting more beautiful and floating ice bergs can be seen all over the lake making it an exceptional case in Indian Himalayas. Trek starts from Sissu in Lahaul Spiti District of Himachal Pradesh. The place is just beneath two mighty mountains of Pir Panjal Range (longest lake in Himalayas) with spectacular landscapes on Manali-leh Highway. But the waterbody and Sissu are not only special attraction on the trek but Stone architect of Gaddi Hut on the route is very interesting and rarely seen thing on any trek.