The Great Himalayan region is one of the few remaining isolated and inaccessible areas in the world today. Some high valleys in the Great Himalayas are occupied by small clustered settlements. Extremely cold winters and a short growing season limit the farmers to one crop per year, most commonly potatoes or barley. The formidable mountains have limited the development of large-scale trade and commerce despite the construction of highways across the mountains linking Nepal and Pakistan to China. Older trails, which cross the mountains at high passes, also have limited trade and are open only during the summer months.