Apparatus and methods for creating a downhole impact. The apparatus may be an impact tool operable to be coupled between portions of a tool string conveyable within a wellbore extending into a subterranean formation. The impact tool may include a housing, a chamber within the housing, a piston slidably disposed within the chamber and dividing the chamber into a first chamber volume and a second chamber volume, and a shaft connected with the piston and axially movable with respect to the housing. The first chamber volume may be open to a space external to the housing and the second chamber volume may be fluidly isolated from the space external to the housing. The piston may be maintained in a predetermined position within the chamber to maintain pressure within the second chamber volume appreciably lower than pressure within the first chamber volume while the impact tool is conveyed along the wellbore.