Bathing has been a popular practice for thousands of years. The first documented plumbing system dates circa 3300 BC with the discovery of copper water supply lines unearthed beneath a palace in the Indus River Valley in India. Romans were encouraged to bathe daily in public baths around 500 BC; accommodations consisted of enormous marble bathtubs complete with lead or bronze piping. Modern bathtubs installed in houses were a nineteenth-century innovation, with cast-iron bathtubs gaining popularity in the 1880s. By the early 20th century it became commonplace for houses to have dedicated bathrooms fitted with a bathtub.