Most of Sikkim, including Gangtok, is underlain by Precambrian rocks which contains foliated phyllites and schists; slopes are therefore prone to frequent landslides. Surface runoff of water by natural streams ("jhora") and man-made drains has contributed to the risk of landslides. According to the Bureau of Indian Standards, the town falls under seismic zone-IV (on a scale of I to V, in order of increasing seismic activity), near the convergent boundary of the Indian and the Eurasian tectonic plates and is subject to frequent earthquakes. The hills are nestled within higher peaks and the snow-clad Himalayan ranges tower over the town from the distance. Mount Kanchenjunga (8,598 m or 28,208 ft)—the world's third-highest peak—is visible to the west of the city. The existence of steep slopes, vulnerability to landslides, large forest cover and inadequate access to most areas have been a major impediment to the natural and balanced growth of the city.