Wilkes-Barre was founded in 1769 and formally incorporated in 1806. The city grew rapidly in the 19th century after the discovery of nearby coal reserves and the arrival of hundreds of thousands of immigrants who provided a labor force for the local mines. The coal mining fueled industrialization in the city, which reached the height of its prosperity in the first half of the 20th century. Its population peaked at more than 86,000 in 1930. Following World War II, the city's economy declined due to the collapse of industry. The Knox Mine disaster accelerated this trend after large portions of the area's coal mines were flooded and could not be reopened. Today, the city has around half of its peak population of the 1930s, making it the largest city in Luzerne County and the 13th-largest city in Pennsylvania.