( CNN ) -- As divers searched frigid waters off South Korea in low visibility , hoping to save hundreds of passengers , a dominant theory began to emerge about how the ferry sank . It most likely struck something in the water , said Peter Boynton , a retired U.S. Coast Guard captain . `` The speed with which this ferry began to list and then roll over on its side suggests significant damage , most likely causing major flooding that would cause a vessel of this size -- almost 500 feet long -- to quickly roll onto its side . That 's very likely the result of significant damage , '' he said . Some passengers reported hearing a loud bang before the ship began sinking . That could be from cargo shifting or `` some other internal damage , '' Boynton told CNN 's `` New Day . '' `` But it does sound , from initial reports , it was more likely that something was struck . '' When the ship left Seoul , it traveled through fog , which may have put it off course , said Mary Schiavo , former inspector general for the Department of Transportation . `` So if they hit something , that would have meant they were out of the channel , which is quite easy to do , '' Schiavo said . 'We are not dead yet ' passengers texted . But the South Korean Oceans and Fisheries Ministry said Thursday that the ferry did not deviate significantly from its intended route . The agency approved the ferry 's intended route , and `` there was no huge difference between their plan and the actual track chart , '' spokesman Nam Jae Heon said . Schiavo said other possibilities include engine failure or an explosion , particularly in the engine room . `` But that probably alone would n't account for the sinking this quickly . It probably was something else that happened , '' she said . Making matters worse , the ferry carried dozens of vehicles . Once an auto deck is breached , `` it 's typically open to very significant flooding , '' Boynton said . That could explain `` why the ferry in just a matter of hours began to roll onto its side so quickly . '' Coast guard and navy ships , as well as fishing boats , rushed into the area . The challenge ahead . For rescue divers , a combination of factors makes saving people especially difficult : very cold waters , strong currents and low visibility , made worse by nightfall . `` The underwater challenges are very , very significant and pose , I would think , tremendous risk for the people who I 'm sure are doing their best to help , '' Boynton said . For the passengers , the most immediate danger is the cold . `` Pretty much everyone we saw was wearing a life jacket , '' journalist Andrew Salmon reported on CNN International . `` So the concern is hypothermia . If you 're not picked up within two hours , you 're in significant danger -- your body core goes cold . '' Can they survive in air pockets ? Some of the rescued passengers report that when the ship began to sink , they were told to jump into the water immediately -- and not to take time to get into life boats . Sometimes after a breach , as the water begins gushing in , `` there 's a sucking , there 's a motion , that just makes it impossible to fight , '' Schiavo said . `` So the order to abandon ship might have indicated that . ... It 's almost like a suction that occurs when the water starts coming on , and you ca n't fight it . '' But other passengers said they were told to stay on the ship . Sometimes , `` conflicting commands '' are given , Schiavo said . `` There can be a lot of confusion in an event like this . ''
South Korean oceans ministry : The ship did not deviate much from its intended course . Some passengers reported hearing a noise before the ship sank . The ship had traveled through fog , but conditions were clear at the time of the sinking . The most immediate concern to passengers is safety .