Jane’s:
North Korea is constructing a fortified structure near the port city of Sinpo that will have what appears to be two covered docks (pens) that could shelter ballistic missile submarines (SSBs).
Located 2.25 km south of the Sinpo shipyard, close to the Mayang-do Naval Base on the country’s east coast, the new base may be the largest active military building project in North Korea at the moment.
Commercial satellite imagery shows construction of the base began sometime between August 2009 and November 2012. Much of the harbour seen in 2009 (an area covering some 6,000 m²) had been blocked off by a sea wall and filled in by November 2012. Spoil from the surrounding hills was likely used as filling material.
The harbour area was nearly filled in by October 2013, excavation of the docks was progressing, and the construction of a new peer had begun. The docks were taking shape by 24 July 2014, with the North Koreans getting ready to pour their concrete bases.
By mid-2015 the steel structures over the docks were being set and some concrete slabs had been laid to form the roofs of the pens. Imagery from 13 May 2015 shows the two pens are approximately 150 m long, 10 m wide, and 14 m apart.
Satellite imagery from 8 May 2016 revealed that construction on the pens had progressed to the extent that portions of them were being covered with earth. Construction was still ongoing on the front of both pens and a barge was tied to the seawall. The new pier, now 137 m long and 13 m wide, was nearing completion.
