Did they ever figure out what caused the loss?
My understanding is the hatch bungs came loose and they lost tbe main hatch. She started taking water very fast. There was another ship behind her that had her on radar. She just disappeared.
The ore was loaded through hatchways located top side. On October 31 routine damage was noted during an inspection and were scheduled for repair after the 1975 shipping season. The hatch covers were not sealed properly and were therefore not water tight, thus allowing water to enter the cargo areas. Once water entered it could migrate throughout the hold. There was no way to determine if flooding was occurring in the cargo bay until the ore was saturated, much like a sponge. Throughout the storm the ship was probably taking on water in the cargo hold though the hatches. Increased water loading, and the lower load line, made the ship sit lower in the water, allowing more water to board the ship. Eventually the "bow pitched down and dove into a wall of water and the vessel was unable to recover. Within a matter of seconds, the cargo rushed forward, the bow plowed into the bottom of the lake, and the midship's structure disintegrated, allowing the submerged stern section, now emptied of cargo, to roll over and override the other structure, finally coming to rest upside-down atop the disintegrated middle portion of the ship" (Marine Accident Report SS Edmund Fitzgerald Sinking in Lake Superior). This sequence of events would lead to a rapid sinking, with no time to make a distress call or attempt life-saving operations. The conditions of the recovered lifeboats support this in that they appear to have been torn from their storage racks.
One theory is that the Fitzgerald had partially grounded itself on Six Fathom Shoal and began taking on water - although it's highly likely that water has coming in already because of loose hatches.
This happened around 3:15PM. The captain of the Arthur M Anderson later testified that he commented (to someone on the bridge) that the Fitzgerald was closer to Six Fathom Shoal than he'd want to be.
There are a number of theories. Here's a link where they're briefly mentioned:
http://www.ssefo.com/info/theories.html