Željava Air Base

Deliberate and systematic destruction during the Yugoslav Wars. As the Yugoslav People's Army (JNA) retreated from the area, it destroyed the base to prevent it from being used by the newly formed Croatian or Bosnian armies. The JNA's military engineers used approximately 56 tons of explosives to detonate pre-planned demolition charges. The explosions destroyed the runways and caused catastrophic collapses within the underground tunnel complex, rendering the entire facility permanently inoperable.

The site is completely abandoned and in a state of ruin. The surface runways are cracked, buckled, and overgrown with vegetation. The underground tunnels are heavily damaged, partially collapsed, flooded, and extremely dangerous. The complex is contaminated with hazardous materials, including PCBs, asbestos, and remnants of toxic fuels and chemicals released during its destruction. The surrounding area, particularly along the border between Croatia and Bosnia and Herzegovina which runs through the complex, remains heavily contaminated with unexploded landmines and ordnance. Despite the extreme danger, the site has become a destination for 'dark tourism', urban explorers, and military history enthusiasts. The border police of both nations patrol the area to prevent illegal crossings and to deter visitors from entering the hazardous zones. Parts of the external runways are occasionally used for unofficial car races and driver training.

Željava Air Base, code-named 'Objekat 505', was the largest and most expensive military construction project in the history of the Socialist Federal Republic of Yugoslavia, with costs estimated to be between $4.5 and $8.5 billion USD. Built between 1957 and 1968, it was one of the largest underground military airfields in Europe. Its primary purpose was to serve as a highly protected, strategic command center and air base, designed to withstand a direct 20-kiloton nuclear blast. The heart of the base was the 'Klek' underground complex, a network of tunnels over 3.5 km long, which could house, arm, and maintain up to 60 MiG-21 fighter jets and accommodate 1,000 personnel. The complex was self-sufficient for 30 days, with its own power generators, water sources, and advanced air filtration systems. It featured four entrances protected by 100-ton pressurized blast doors. On the surface, the base had five runways. It was home to elite Yugoslav Air Force units, including the 117th Fighter Aviation Regiment, and played a crucial role in Yugoslavia's air defense strategy during the Cold War. In 1991, at the start of the Yugoslav Wars, the base was used for air operations against Croatia and Bosnia and Herzegovina.

There are no concrete or funded plans for reopening or redeveloping Željava Air Base. Over the years, numerous proposals have been discussed, including converting it into a cargo airport, a commercial data center, a military training facility, or a tourist attraction/museum. However, all prospects face insurmountable obstacles:
1. **Prohibitive Costs:** The cost of de-mining the vast area, safely removing toxic and hazardous materials, and repairing the massive structural damage to both surface and underground facilities is estimated to be in the billions of euros, making any project economically unviable.
2. **Bi-national Jurisdiction:** The base straddles the international border between Croatia and Bosnia and Herzegovina, creating significant legal, political, and logistical challenges for any joint development project.
3. **Environmental Contamination:** The severe contamination poses a major environmental and health risk that would require a complex and expensive cleanup operation.
Due to these factors, the site is expected to remain in its current derelict state for the foreseeable future.