Oh !
Simple chemistry theory
They may using colorless gas heavier than the Air, so it can maintain the pressure from the boat
Such as…
Sulfer Hexaflouride ( SF6 )
IUPAC name sulfur(VI) fluoride
colorless, odorless gas
Consisting of six fluorine atoms attached to a central sulfur atom
But if you put some object with less Area ex. Pen or Pencil
Simply it goes down