When was the last time aliens visited the earth?
There is no evidence that aliens have ever visited the earth. There have been many claims including UFO sightings, strange atmospheric patterns, ancient buildings showing characteristics outside of human capabilities at that time. But all of these have been refuted and there is no credible evidence of alien visitation.
Instruction: When was the last time aliens visited the earth?
Context: 
Response: There is no evidence that aliens have ever visited the earth. There have been many claims including UFO sightings, strange atmospheric patterns, ancient buildings showing characteristics outside of human capabilities at that time. But all of these have been refuted and there is no credible evidence of alien visitation.