I was rereading the thriller bark when I arrived at this Chapter. Before the arc there is a legend that hundreds of ship disappear from florian triangle, once moria is introduced it can be assumed that this was moria's doing however the chapter clarify that the legend is indeed true "10 years ago".
Obviously from the silhouette we can easily assume that this is a sea monster,, But the wording seems to imply that this is ancient and deep mystery which obviously connected to the "black Century". If we assume that this a creature then the silhouette is its body, but why is not moving?. this could mean it is dead or simply in hibernation.. Finally the chapter imply that this creature is moving until 10 years ago, which is the start of the series.
Other than its connection in the black century,, I think this creature or whatever it is is connected to Shanks, Well if it is a creature then he probably killed it or injured to the point of hibernation. this may have been one of the reason he become a yonkou.
On the other note,, This silhouette can be a machine or simply the ancient weapon PLUTON, however base on the series it seems that the pluton is in alabasta,,
give me your thoughts,thanks