A vessel, such as a wing-integrated aircraft fuel tank. The vessel can include a first surface portion, a second surface portion spaced apart from the first surface portion, and a core positioned between the first and second surface portions. The core can be sealably connected to the first and second surface portions and can be positioned to carry a load from at least one of the first and second surface portions to the other. The core can include a plurality of cells separated by cell walls, at least some of which have wall openings positioned to provide liquid communication between adjacent cells.