Hello how may I help you? What is the longest thing in the world? I'm sorry but that question is too vague. I would like to know what the longest thing in the world is What do you mean by thing? Can you be more specific? I suppose I mean anything so long as it's the longest thing in the world The Son Doong Caves hold a mighty record. Is that the longest thing ever? You have to be specific. I specifically would like to know what the longest thing in the world is There are many 'thing's on this planet. You have to be specific.