The lakes of Bhutan comprise its glacial lakes and its natural mountain lakes. Bhutanese territory contains some 2,674 high altitude glacial lakes and subsidiary lakes, out of which 25 pose a risk of GLOFs. There are also more than 59 natural non-glacial lakes in Bhutan, covering about . Most are located above an altitude of , and most have no permanent human settlements nearby, though many are used for grazing yaks and may have scattered temporary settlements. Only four lakes are below an altitude of : the temperate Ho Ko Tsho in Punakha District at ; Luchika in Wangdue Phodrang at ; Buli in Zhemgang at ; and the subtropical Gulandi in Samdrup Jongkhar at . As phenomena of nature, all lakes in Bhutan are believed to be inhabited by spirits. A handful of lakes in Bhutan are particularly sacred, most often connected to lives of Buddhist saints Guru Rimpoche and Pema Lingpa. For example, Membar Tsho ("Burning Lake"), in the Tang Valley near Bumthang, is heavily associated with Guru Rimpoche, who brought Buddhism to Bhutan and discovered his first terma in the lake in 1475. Most natural lake names are denoted with the word tsho, meaning "lake" (Dzongkha: མཚོ་; Wylie: mtsho). They may be identified as an individuals, pairs, or groups of lakes. Glaciers and glacial lakes Bhutan contains some 2,674 glacial lakes. Some glacial lakes, such as Thorthormi Lake in Lunana Gewog, are not a single bodies of water but collections supraglacial ponds. Most glacial lakes identified as potentially dangerous feed into the Manas River and Puna Tsang (Sankosh) River water systems of north-central Bhutan. During a GLOF, residents of nearby downstream villages may have as little as twenty minutes to evacuate; floodwaters from one 1994 GLOF at Luggye lake took about seven hours to reach Punakha, some downstream. Where glacial movement temporary blocks riverflows, downstream areas may be threatened by glacial lake outburst flood ("GLOFs"). Although GLOFs are not a new phenomenon in Bhutan, their frequency has risen in the past three decades. Significant GLOFs occurred in 1957, 1960, 1968 and 1994, devastating lives and property downstream. According to the Bhutan Department of Energy however, the majority of rivers in Bhutan are more susceptible to fluctuation with changing rainfall patterns than to flooding directly attributable to glacier or snow melt. For public safety, these glaciers and glacial lakes are maintained by the Ministry of Economic Affairs' Department