The Mountain, also known as the Cursed Mountain, is a huge mountain situated on the southern side of the Island, considerable close to the Tower. The mountain was believed to be cursed as it seemed dead people were resurrected at the mountain, but this was proved false as the hallusinations caused by mountain-altitude sickness was the true reason for this.
The Mountain is man-made as Kouichi explains it in Chapter 92. It's composed of various materials including melted plastic. It's unknown how high it is, but it took Akira's Group almost a day to climb up the top and Kouichi stated that it is more than a thousand meters.
Cursed Mountain arc
Tower of Death arc
On chapter 174, it has been revealed that it used to call "Tanho Mountain".