Speculation has been running high since Wednesday what caused a ferry carrying 475 passengers to sink off the southwest coast, leaving nearly 290 people including many youngsters missing.
The Sewol was scheduled to leave Incheon at 6:30 p.m. Tuesday but did not make it out until about 9 p.m. due to thick fog.
Rescue forces said it is possible the captain shifted course to make up for lost time and ended up hitting a rock.
A spokesman for ferry operator Chonghaejin Marine told reporters the Sewol did not stray "too far" off its safe course, but maritime experts said the ferry could have hit rocks by shifting its course even slightly in thick fog.
Other experts agreed. Rescued passengers said they heard a loud bang at the front of the ferry, strongly supporting the collision theory.
Chung Yong-hyun at the Korea Diving Industry Institute said, "A large hole caused by a submerged rock is the most probable reason why enough water poured into the ferry to cause it to capsize."
But Hwang Dae-shik at the Maritime Rescue and Salvage Association said, "Based on accounts from local fishermen and rescue workers, the waters off Jindo, South Jeolla Province," where the ferry sank, "do not have a lot of submerged rocks. We need to take a close look at other possible causes."