Mine sweeping for the Wonsan landing commenced on 8 October, when Task Group 95.6, commanded by Captain Spofford, began assembling for the mission of clearing a path ahead of the 250-ship armada bringing the 1st Marine Division and other units of X Corps. It had been known for a month that the waters of the east coast were dangerous for navigation. The first mine was discovered off Chinnampo on the west coast on 7 September, and four days later Admiral Joy ordered the United Nations Blocking and Escort Force to stay on the safe side of the 100-fathom line along the east coast. But it was not until 26 and 28 September that more definite information was acquired the hard way when the U. S. destroyer _Brush_ and the ROK mine sweeper _YMS 905_ were damaged by east coast mines.[68] [68] CinCPacFlt _Interim Evaluation Report No. 1_, VI, 1090. On the 28th ComNavFE issued his OpnO 17-50 covering operations of mine sweepers in Korean waters. The herculean task awaiting the 12 available American vessels of this type may be judged by the fact that more than a hundred had been employed off Okinawa in World War II. Although the exact date remained unknown, it was a safe assumption that North Korean mining activities, beginning in late July or early August, were speeded by the Inchon landing, which aroused the enemy to the peril of further amphibious operations. Russian instructors had trained Korean Reds at Wonsan and Chinnampo in the employment of Soviet-manufactured mines. Sampans, junks, and wooden coastal barges were used to sow a field of about 2000 in the harbor and approaches to Wonsan.[69] [69] _Ibid._, VI, 1088–1089; Smith, _Notes_, 404; Karig, _Korea_, 301. See also ADVATIS Rpt 1225 in EUSAK _WD_, 24 Oct 50. Captain Spofford’s TG 95.6 commenced its sweep off Wonsan on 10 October after a sortie from Sasebo. Unfortunately, the three large fleet sweepers, _Pledge_, _Pirate_, and _Incredible_, were not well adapted to the shallow sweeping necessary at Wonsan. More dependence could be placed in the seven small wooden-hulled U. S. motor mine sweepers _Redhead_, _Mocking Bird_, _Osprey_, _Chatterer_, _Merganser_, _Kite_, and _Partridge_, which were rugged even though low-powered. Spofford’s two big high-speed sweepers, _Doyle_ and _Endicott_, had their limitations for this type of operation; and the nine Japanese and three ROK sweepers lacked some of the essential gear.[70] [70] CinCPacFlt _Interim Evaluation Report No. 1_, VI, 1004; Dept Army, Joint Daily Situation Report (D/A Daily SitRpt) 105; Karig, _Korea_, 311–314.