Yugoslavia's biggest underground airport lies in ruins.

Since 1973, this block has been home to the most concentrated collection of murals in San Francisco.

The last remaining relic of San Francisco's glittering 1915 Panama Pacific Exposition.

In all of the 29 years it was in operation, no prisoners ever escaped successfully... or so they claim.

Concrete strongholds stand guard over the Golden Gate.

The oldest Naval cemetery on the West Coast.

The former Naval shipyard is open for exploring - mostly.

Hand-powered race of alternative forms of transportation.

340 bicycles, 65 feet high.

The world's oldest known salt mine.

The famous aviator's final resting place is a study in natural beauty.

Aqueducts have dramatically changed this old lake, now home to tufa towers and its very own species of tiny brine shrimp.

The ghostly remains of what was once the world's richest copper mine.

A theme park that suffered massive flooding during Katrina was abandoned, and now proves irresistible to trespassing explorers.

This WWII-era military installation once held an entire city within its walls - now, its walls are falling down.