A mountain is an elevated portion of the Earth’s crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited summit area, and is usually higher than a hill, typically rising at least 300 metres (980 ft) above the surrounding land. A few mountains are isolated summits, but most occur in mountain ranges. Mountains are formed through tectonic forces, erosion, or volcanism, which act on time scales of up to tens of millions of years.
Once mountain building ceases, mountains are slowly leveled through the action of weathering, through slumping and other forms of mass wasting, as well as through erosion by rivers and glaciers. High elevations on mountains produce colder climates than at sea level at similar latitude. These colder climates strongly affect the ecosystems of mountains: different elevations have different plants and animals. Because of the less hospitable terrain and climate, mountains tend to be used less for agriculture and more for resource extraction, such as mining and logging, along with recreation, such as mountain climbing and skiing.
Here are the top 20 tallest mountains in the world.
| Rank | Mountain | Country | Height |
| 1. | Mount Everest | Nepal, China | 8,849 m (29,032 ft) |
| 2. | Godwin Austen (K2) | Pakistan, China | 8,611 m (28,251 ft) |
| 3. | Kangchenjunga | Nepal, India | 8,586 m (28,169 ft) |
| 4. | Lhotse | China, Nepal | 8,516 m (27,940 ft) |
| 5. | Makalu | Nepal, China | 8,485 m (27,838 ft) |
| 6. | Cho Oyu | China, Nepal | 8,188 m (26,864 ft) |
| 7. | Dhaulagiri I | Nepal | 8,167 m (26,795 ft) |
| 8. | Manaslu | Nepal | 8,163 m (26,781 ft) |
| 9. | Nanga Parbat | Pakistan | 8,126 m (26,660 ft) |
| 10. | Annapurna I | Nepal | 8,091 m (26,545 ft) |
| 11. | Gasherbrum I (K5) | Pakistan, China | 8,080 m (26,510 ft) |
| 12. | Broad Peak | Pakistan, China | 8,051 m (26,414 ft) |
| 13. | Gasherbrum II (K4) | Pakistan, China | 8,035 m (26,362 ft) |
| 14. | Shishapangma/Gosainthan | China | 8,027 m (26,335 ft) |
| 15. | Gyachung Kang | Nepal, China | 7,952 m (26,089 ft) |
| 16. | Gasherbrum III (K3) | Pakistan, China | 7,946 m (26,070 ft) |
| 17. | Annapurna II | Nepal | 7,937 m (26,040 ft) |
| 18. | Gasherbrum IV (K3) | Pakistan | 7,932 m (26,024 ft) |
| 19. | Himalchuli | Nepal | 7,893 m (25,896 ft) |
| 20. | Distaghil Sar | Pakistan | 7,884 m (25,866 ft) |
