The Borealis was the last ship created by mankind and was supposively the first and only ship capable of crossing the gap without the need for a bridging device (Tachyon Jump Gate). Not many facts can be verified or denied about this ship save that it's firmly believed to be the cause of mankind's destruction and rendering the Sol System "un-sailable."
The Borealis is a ship deeply seeded in legend and hearsay. Some argue that the ship never existed and that Mankind is simply in a dormant state. Others believe it's a true honest to goodness ship, massive in scale, the biggest vessel ever seen and that it's still out there and feeds off the souls of the dead. Considering the sparseness of humans in the Expanse, many feel everything is conjecture; though, those that seek fortune and glory equate the Borealis to a hidden treasure that one would simply need to find to make profit from. It's a whisper on every breath of every barman in every pub in all the known and unknown galaxy.