Deep underground on a lush green island, Finland is preparing to bury its highly-radioactive nuclear waste for 1,00,000 years -sealing it up and maybe even throwing away the key.

Tiny Olkiluoto, off Finland's west coast, will become home to the world's costliest and longest-lasting burial ground, a network of tunnels called Onkalo -Finnish for ''The Hollow''.

Olkiluoto nuclear power plant on the island of Eurajoki, western Finland / Source: AFP

Countries have been wrestling with what to do with nuclear power's dangerous by-products since the first plants were built in the 1950s.

Most nations keep the waste above ground in temporary storage facilities but Onkalo is the first attempt to bury it for good. Starting in 2020, Finland plans to stow 5,500 tons of nuclear waste in the tunnels, over 1,380 feet below the Earth's surface.

Already home to one of Finland's two nuclear power plants, Olkiluoto is now the site of a tunnelling project set to cost up to $4 billion until the 2120s, when the vaults will be sealed for good.

Olkiluoto is the site of a tunnelling project set to cost up to $4 billion until the 2120s / Source: AFP

“This has required all sorts of new know-how,“ said Ismo Aaltonen, chief geologist at nuclear waste manager Posiva, which got the green light to develop the site last year.

The waste is expected to have lost most of its radioactivity after a few hundred years, but engineers are planning for 100,000, just to be on the safe side.

At present, Onkalo consists of a twisting 5-km tunnel with three shafts for staff. Eventually the nuclear warren will stretch 42 km. The temperature is cool and the bedrock is extremely dry—crucial if the spent nuclear rods are to be protected from the corrosive effects of water.