A plug for an inspection hole of a gas turbine engine is disclosed. The sub-ambient pressure cavity has a cavity floor, a plurality of sides and a depth that progressively varies between a point on one of the sides and a corresponding point on an opposing side. The ink makes it possible to perform stable jetting operation for a long term without causing any deterioration of metal members of a discharge head due to the metallic corrosion and the elution of nickel into the ink.