The relocation of a U.S. Marine Corps base to a less-crowded area of the southern Japanese island of Okinawa will take more than twice as much money and time as previously estimated because of the need to stabilize the reclaimed land it will be built on, Japan's government said Wednesday.

The Defense Ministry said the relocation of U.S. Marine Corps Air Station Futenma from densely populated Ginowanto Henoko on Okinawa's eastern coast will cost 930 billion yen ($8.5 billion) and take 12 years, pushing its completion into the 2030s. That adds more than a decade to the plan, which has already been delayed by more than 20 years because of local opposition and other reasons.

Under an earlier plan agreed to by Tokyo and Washington in 2013, construction was to cost about 350 billion yen ($3.2 billion) and take five years, with completion expected in about 2022.

Most of the additional cost and time is required to stabilize and strengthen reclaimed land off the coast of Henoko that will be used for runways, the ministry said. It presented its new estimate to a panel of Japanese experts discussing the relocation plans.

Experts have found parts of the sea bottom at the planned reclamation site to be “as soft as mayonnaise” and needing to be reinforced.

Many Okinawans oppose the relocation, saying the base should be entirely removed from Okinawa. The heavy U.S. military presence on Okinawa has been a source of a long-running conflict between the island and Washington and Tokyo.

Opponents also say the relocation plan should be scrapped for environmental reasons because the site is a habitat for dugongs and corals.