While Joplin was first settled for lead mining, zinc, often referred to as "jack", was the most important mineral resource. As railroads were built to connect Joplin to major markets in other cities, it was on the verge of dramatic growth. By the start of the 20th century, the city was becoming a regional metropolis. Construction centered around Main Street, with many bars, hotels, and fine homes nearby. Joplin's three-story "House of Lords" was its most famous saloon, with a bar and restaurant on the first floor, gambling on the second, and a brothel on the third. Trolley and rail lines made Joplin the hub of southwest Missouri. As the center of the "Tri-state district", it soon became the lead- and zinc-mining capital of the world.