Archaeology really is a career in ruins.
Why this joke works
A Career in Archaeology
A career in ruins normally sounds like a professional disaster. Archaeology makes ruins a possible workplace instead. The sentence never changes its wording; the listener changes which meaning of ruins applies. Read it as a sincere observation about choosing a profession. A gloomy delivery can obscure the affectionate subject joke, while a calm voice leaves room for the unexpectedly positive interpretation.
A closer look
Context reverses the verdict
The phrase needs its subject at the beginning. Without archaeology, the listener has no reason to reinterpret professional failure as working among old remains. This makes it a compact example of context changing a sentence's meaning. It suits a museum conversation better than a moment when someone is worried about their actual job prospects.
Fact behind the joke
More than ancient buildings
Archaeology studies material remains left by past people. An artifact can be something people made, altered, or used, so the field extends well beyond ruined buildings. A small everyday object can also provide evidence. The punchline selects one recognizable part of that much broader work.