A hidden network of subterranean passageways lies tangled beneath Indianapolis' bustling City Market.

A medical facility that has been kept frozen in time.

A suspended garden, now gone, once hung in the middle of this abandoned church.

An abandoned relic of the city's failed experiment with below ground transit.

Prehistoric animals decorate this popular outdoor park.

Limestone caves with spectacular showcase chambers.

The largest underground church in Europe.

Ornate limestone building now filled with geologic specimens, and more.

The world's longest known cave system.

The seamy, underground secrets of Portland's criminal past.

Outstanding collection of 19th century medical instruments includes a life-size figure cast from as many as 200 cadavers.

One man's vision and a labor of love of many.