South Asia is the center of one of the great ancient civilizations of the world—the Indus Valley civilization in present-day Pakistan and northwestern India. Copper metallurgy, stone bead making, and seal carving develop in conjunction with the elaboration of ceramics, some decorated with an early form of writing. By the middle of the third millennium B.C., this civilization features a settled urban culture and extensive trading contacts with cultures to the west. Seals provide evidence of a still undeciphered writing system. Other regional cultures are far less developed. The decline of the Indus Valley civilization is attributed to environmental change.