Northumberland is a black submarine who sleeps during the day and works during the night.
He is hardly ever awake in front of the tugs, except for when Theodore and Hank saw him on their night shift. Northumberland is a very quiet sub too and hates loud noise and bright lights. He also helps the "Salvage Team" (Theodore, Rebecca and Shelburne) to find great things under the water. He never likes to disappoint his friends, which can sometimes get him or his friends into danger. He once broke his propeller in an underwater rock slide and showed Hank where a sunken ship was. Northumberland is also a big brother-like figure to Pugwash, but was once scared that she would bump him. Northumberland is always a great help to the tugs and the Salvage Crew and whether he's awake or half-awake, he is always willing to be some assistance to anyone in the Big Harbor. Northumberland lives at the Navy Yards and helps Foduck with his safety duties, but he is always welcomed anywhere.
- ERTL models (retired)
- Northumberland is named after the Northumberland Strait, a body of water that lies between New Brunswick, Nova Scotia mainland, and Prince Edward Island.