The hidden tower
This tower can be found by swimming round from northern Tirisfal Glades, staying close to the shoreline so as not to induce fatigue. Its purpose is unknown but it is possible it is an old model of a potential way of reaching the Blood Elf starting area, before the Burning Crusade was completed.
Other theories are that it is part of upcoming content to do with the next expansion.
Further to this tower if you swim around even further you can prove that the Blood Elf starting area is in fact an instance, not part of of the main Azeroth world. This can be seen in the image where she is swimming in the sea, but is allegedly in the middle of the Ghostlands.