Mohenjo-Daro was one of the largest settlements of the Indus Valley Civilization. Its most remarkable feature is the 'Great Bath,' a large rectangular tank that is often considered the first public water tank in the ancient world. It was made of fine bricks and sealed with a layer of natural bitumen (tar) to make it waterproof. The sophistication of the city, which featured a grid-like street plan and the world's first known urban sanitation system (with covered drains and indoor toilets), suggests a highly organized society with a strong focus on hygiene or ritual purity. Unlike Egypt or Mesopotamia, no grand palaces or temples have been found, leading some archaeologists to believe the Indus Valley may have been a more egalitarian society than its contemporaries.