Born from bubble-era optimism, this visionary building would have housed 30 million people — and swallowed the entire Yamanote Line loop.

The brainchild of Dr. Toshio Ojima, an architect, environmental engineer and urban scientist at Waseda University, the proposed arcology — the proper name for megastructures meant to curb urban sprawl — would have taken anywhere from 100 to 150 years to build. The estimated cost in 1991, the year Ojima made his proposal? A mere ¥3,000,000,000,000,000 — that’s ¥3 quadrillion.