North Korea has carried out new construction this year at a massive underground complex long rumored to be a secret nuclear site, an NK Pro investigation has found, reviving concerns about the purpose of the remote facility.

Located in a mountainous area some 75 miles northeast of Pyongyang, the Hagap (하갑) Underground Facility first became publicly known in 1998, with U.S. intelligence reportedly concluding that it could be used for the country’s nuclear weapons program as soon as 2003.

However, the DPRK has never declared Hagap as part of its nuclear program or opened the site to