Complex sentence: The landfill was opened in 1947 as a temporary landfill, but eventually became New York City's principal landfill, in the second half of the 20th century, and it was once the largest landfill, as well as human-made structure, in the world.
Split into simpler sentences: The landfill was opened in 1947 as a temporary landfill but became New York City's principal landfill in the second half of the 20th century. It was once the largest landfill, as well as human-made structure, in the world.