The phrase "infernal machine" is typically used to describe a device or contraption that is dangerous or destructive in nature. There are several synonyms that can be used in place of "infernal machine", including explosive device, bomb, time bomb, booby trap, landmine, and incendiary device. These terms all describe devices that are designed to cause harm or destruction, and can be used interchangeably with "infernal machine". In certain contexts, the term "infernal machine" may carry a greater sense of mystery or danger, while other words like "bomb" are more straightforward and commonly understood. Ultimately, the choice of synonym will depend on the context and intended tone of the piece of writing or conversation.