The Border Sea was an ocean previously under the rule of Drowned Wednesday. After Wednesday transformed into a leviathan and submerged into the sea, the sea overflowed nine-tenths of all land in the demesne including Port Wednesday. A new port was partially prepared on Wednesday's Lookout at the top of the mountain Port Wednesday was built on before this deluge. Any of the buildings that were submerged were transformed into ships, which thereafter sailed the Border Sea along with Wednesday’s original fleet of 49 ships. A unique aspect of the Border Sea is that anything that is lost accidentally, but not stolen, will appear in it.
Few land masses have been mentioned on the Border Sea, the ones other than Port Wednesday being The Triangle and Swirleen Deep. The Triangle is the top most part of Wednesday's Lighthouse as was before the deluge and takes the appearance of the tip of a pyramid, with the highest point removed to leave an area the size of a tennis court in order to attach a ring to which ropes are secured in. Ships can then secure themselves to these ropes and remain at The Triangle indefinitely.
It is mentioned that the ceiling of The Border Sea is higher than in other parts of the House, as Arthur was fearful of crashing into it while being carried by Wednesday's Dawn before realising this fact.
The Border Sea extends into any body of water in the Secondary Realms, and therefore can be a means of travelling anywhere. The Line of Storms, a series of vicious lightning storms, guard the entrance into the sea; only an express invitation into the Sea will prevent the Line from killing people instantly. Whether the Line Of Storms acts as a wall of the House akin to the mountains in The Great Maze is probable, but it is not explained fully, i.e. We are unsure if the line of Storms appears only where an individual intends to cross from the House to the Secondary Realms, or whether the Line is stationary and one must travel to it. It is probable that the former is more accurate.
Raised Rats live in the Border Sea and pilot their steamships. Pirates also live here, of whom the most formidable was a mortal pirate who somehow passed the Line of Storms into the Border Sea, Elishar Feverfew who was eventually killed by Arthur with the help of Suzy and Leaf.
At the end of Drowned Wednesday, Wednesday’s Dawn informed Arthur that much of the Sea had extended into the Void of Nothing, and since Wednesday used all of the Key’s power keeping her shape, had not reined it in. As Arthur does not wish to use the Keys unless absolutely necessary, he left this task to his Steward Dame Primus. As of Superior Saturday, the Sea has yet to be brought under control. In Lord Sunday it is revealed to be destroyed by a wave of nothing. However, it is assumed that Lord Arthur plans to rebuild it as it was before the errosion by Nothing occured.