Fast currents, adverse weather conditions, including rain and strong gusts of wind
Rescue divers' attempts to enter into the sunken ferry have failed repeatedly since the search-and-rescue operations began three days ago.
Rescue teams have opted to search for survivors with divers using underwater paths to get into the vessel.
Some have raised questions whether drilling a hole directly into the vessel to pull out any possible survivors might have been a better option.
But experts say, that was not a viable option.
They say that if there are any survivors inside the now-sunken Sewol-ho ferry, they're using air pockets to stay alive.
If a hole had been drilled from the outside, it would have caused water to rush in, collapsing those air pockets.
They also point out, that welding necessary to generate the hole would have generated carbon monoxide within the ship, which could have been fatal as well.
"Carbon monoxide poisoning could have caused a bigger accident. It's extremely rare to execute rescue operations by drilling a hole into a ship."
To help search divers in the area, some have proposed setting up large commercial ships, such as oil tankers, nearby to block rapid tidal currents.
But experts say this is also not an option, because the direction of tides change four times a day, meaning deploying just one or two wouldn't help.
"At least three ships need to be deployed to safely surround the sunken ship to prevent currents, which would take at least six months."
Then what about lifting the ship by cranes?
Refloating the ship, experts say, will take at least a few weeks, which would be far too late for any survivors, if there are any, onboard.
Park Ji-won, Arirang News.