Technology derived from the ancient civilizations of the Indian subcontinent. Although far less closely studied, the Harappan civilization, which flourished in the Indus Valley (in modern Pakistan) around 4000 years ago, ranks with those of Egypt, Mesopotamia, and China. The ancient town of Mohenjo-Daro covered 85 ha (210 acres), and had an advanced town-planning scheme. In historic times Indian science and technology has made important contributions to Western civilization. Wootz steel (a type of crucible steel made in southern India) was prized by the Romans. The great 8-m (26-foot) iron pillar at Delhi (5th century ad) testifies to the skill of early Indian metalworkers. Early Hindu surgeons such as Susruta performed advanced eye operations and practised lithotomy (surgical removal of a stone from the urinary tract) and plastic surgery. Variolation (a form of immunization) was used for protection against smallpox from at least the 5th century ad. Indian philosophers were skilled in astronomy and mathematics—especially algebra and geometry—and their decimal notation found its way to the West via the Arab world.