This 135-year-old weather station studied the effects of the climate on human health.

This military tunnel dates back to World War II and provides visitors with a little wartime adrenaline.

The only gulag ever to exist in socialist Yugoslavia.

When Croatia became independent of Yugoslavia, the splintering governments left behind these concrete tunnels.

Formerly Yugoslavia's "forbidden island," this sleepy little city still holds the abandoned remains of military strongholds.

This truly palatial work of Communist-era architecture now sits crumbling on a Croatian beach.

The remains of a sunken military ship wreck visible through clear water.

This picturesque medieval monastery guards a small trove of historic treasures.

Large, airy hallways hide within an impenetrable fortress that beckons urban explorers.