Over the last 14 months, one of the most ambitious offshore wind power projects in European history has taken shape off the coasts of Germany and the Netherlands: 30 enormous wind turbines, with an expected total output of 108 Megawatts -- enough to power 120,000 homes -- are being erected, at great cost, in the North Sea. Now the Riffgat offshore wind farm has encountered an inconvenient, and very old, obstacle (or rather, obstacles) on the sea floor: unexploded World War II munitions .

The wind farm, which was built for €400 million ($534 million) by German energy supplier EWE, is located 14 kilometers (nine miles) off the island of Borkum, in Germany, and slightly further from the Dutch island of Schiermonnikoog. It is the first private wind farm project in the North Sea. The project was hampered by a bizarre border dispute between the Netherlands and Germany, but was on time for a scheduled opening this month. Now the discovery of 70-year-old explosives on the seafloor has kept grid operator Tennet from installing a power cable connecting it to land -- meaning the wind farm may not be able to provide power to the mainland until next year.

The project was to be a major milestone for both the German offshore wind industry and Germany's energy transition, or Energiewende , from nuclear and polluting sources of energy to renewable ones. In order to build the turbines, which stand on 70-meter (230-foot) columns, EWE had to use a specially built 132-meter-long ship capable of raising itself above the waves on stilts so that it could assemble the carefully calibrated turbines without being jostled. The turbines can have, at most, 0.1 percent curvature, a limitation which requires extremely precise construction.

An Undersea Graveyard for Explosives

Two weeks ago, Tennet announced that it would not be able to complete the 50-kilometer-long cable connecting the wind farm to the mainland, because explosives were found in the area where the final section of the cable was to be dropped. During World War II, the North Sea was heavily mined, and bombs were often dropped into the sea by British planes on their way back home, in order to save on fuel, creating an undersea graveyard for explosives. German government experts believe 1.6 million tons of old explosives are located in the waters off Germany.

The munitions, according to EWE and Tennet, have been on the map for decades -- the area where the wind farm was built, notably, had to be cleared of 2.7 metric tons of explosives during construction  . But according to Henrike Lau, a spokeswoman for Tennet, the company wasn't aware of the full extent of the munitions until a new, 2012 study was released. Unfortunately, clearing the munitions offers its own logistical challenges. "The salvage operation was made more difficult because of the strong current, which moves the munitions around, and the presence of algae, which make it difficult to see the sea floor," said Lau.

Only a handful of companies have the qualifications and equipment to remove seafloor explosives -- the special ship required for it costs up to €200,000 per day -- and according Lau, the explosives won't be cleared until late August. By that point, the ship that is supposed to lay the cable will already be booked for work elsewhere, until early 2014. Now the plan is to be finished by mid-February. In the interim, Tennet is required to reimburse EWE for 90 percent of the losses incurred by the delay -- costs the Berlin daily Die Tageszeitung recently reported might run as high as €6 million a month. Costs that, of course, may be passed on to German households in the form of grid use fees.