Depends what you mean by UFOs. If you mean 'spaceships with little green men' nobody has proved that such visits have ever taken place, and indeed, given the vastness of space, it is unlikely that they ever will. Some strange visual phenomena have been observed for which there is no explanation as yet; by their very nature no explanation for them exists.
Some people believe that spaceships from other planets have visited Earth, but no one has ever been able to prove it.
probably san fransisco, california
Since no humans have visited Mars, it is unsure. But Mars is often mistaken for UFOs because of how bright the planet is.
There seems to be a growing body of evidence that the Earth is visited by extraterrestrials. But until a time when they land and introduce themselves we may never know for sure.
There is no generally-accepted evidence that this occurred. The "alien visitors" theories concerning UFOs and alleged abductions are scientifically unsubstantiated. It is possible that non-intelligent lifeforms may have reached Earth in the form of organic material thrown into space by collisions of other planets or stars.
Like flying machines on Earth, UFOs are equipted with landing gear.
UFOs are very often seen on Earth, and nobody knows what is inside of one. If there are truly UFOs from space, when they supposedly come to Earth they may already be learning from us.
There are those among us who believe that UFOs have always been hovering nearby.
There is currently no confirmed evidence of extraterrestrial spacecraft visiting Earth. While there have been reported sightings of unidentified flying objects (UFOs), none have been proven to be of extraterrestrial origin.
UFOs Over Earth - 2008 The Fayetteville Incident - 1.1 was released on: USA: 27 October 2008
earth has never visited another planet
Yes, I can. -- Complete list of planets that humans have visited: Earth -- Complete list of other bodies outside of earth that humans have visited: The Moon.