Some 50 ships and helicopters are currently racing against time to rescue the 476 passengers aboard a sinking South Korean passenger ferry.
The ferry's passengers included 324 students and 14 teachers from Danwon High School.
South Korea's disaster control agency said 179 people have been rescued so far, including all of the students and teachers.
One person, identified as Park Ji Young, a female employee of the boat operator, had died.
Authorities have yet to determine the cause of the accident. Witnesses however narrated hearing a loud impact. Minutes later they saw ship listing and quickly sinking.
"We heard a big thumping sound and the boat stopped," one of the rescued passengers told the YTN news channel. "The boat is tilting and we have to hold on to something to stay seated."
Sailing from Incheon, a port west of Seoul, to the resort island of Jeju, the 6,825-tonne Sewol ferry managed to send out a distress signal about 20km (12 miles) off the island of Byungpoong at about 09:00 local time (00:00 GMT) on Wednesday morning before it started sinking, according to various news agencies.
Jeju is 60 miles off the south coast of South Korea.
The rescue team includes 34 naval, coastguard and civilian ships, as well as 18 helicopters. Navy divers were also seen on the scene.
Yonhap news agency reported that South Korean President Park Geun-hye had ordered coast guard commandos to search the entire ship to ensure no-one was left behind.
South Korean television pictures showed the Sewol lying on its port side. Afterwards, the ship completely capsized, with only the forward part of its white and blue hull showing above the water.