Conceptual supertall designs, such as The Illinois by Frank Lloyd Wright (image, right), X-Seed 4000, and Sky City 1000 are probably technologically and architecturally feasible, but are considered by some to be simply too tall to be of any particular use. Critics suggest that the current world economic situation does not require such a massive concentration of people and capital and, after the occurrence of the September 11, 2001 attacks, fear that such towers will almost certainly become terrorist targets