India, April 10 -- With the push of a button, the elevator descends hundreds of meters in seconds into the dark depths of Onkalo.

"We are now at about minus 430 meters (1,411 feet)," muttered geologist Tuomas Pere as he steered a car through a labyrinth of man-made tunnels. "We are driving through 1.9-billion-year-old bedrock."

After decades of construction, the world's first facility for permanently disposing of spent nuclear fuel is set to begin operations in Finland, becoming a final resting place for tons of dangerous radioactive waste.

Construction of Onkalo - which means "cave" in Finnish - began on the west coast in 2004. It sits on the secluded island of Olkiluoto, in a dense wooded area.

The closest town is Eurajoki, about 15...