A seemingly endless concrete line stretching through the French countryside, which sits as a monument to a discarded engineering advancement.

A maze of tunnels filled with haunting remains snake their way beneath the streets of Paris. A town in the USA built on greed. A ring of unique fortifications battered by a mighty weapon. A strange formation of domed structures hovering over the waters off the Florida coastline. But how and why were they ever built at all, and why are they now left abandoned?