The idea that aliens, or extraterrestrial life forms, have visited or are currently visiting Earth is a popular conspiracy theory. Many believe that the government, mostly the United States government, have knowledge of and have possibly been in contact with aliens, but have kept this information hidden from the public. 

One of the most well-known examples of this conspiracy theory is the so-called “Roswell Incident”, which refers to an alleged crash of an extraterrestrial spacecraft in Roswell, New Mexico in 1947. According to the conspiracy theory, the US military covered up the crash and has since been hiding evidence of extraterrestrial life. However, the US military has said that the crash was actually a weather balloon and not a spacecraft. Additionally, some conspiracy theorists believe that the US government has been involved in a secret program known as the “Project Blue Book”, which supposedly aimed to investigate and cover up sightings of UFOs. However, the US Air Force, who conducted the project, have stated that the project was a study that investigated Unidentified Flying Objects reports it concluded that none of the spacecraft were from other planets.

Another popular conspiracy theory is that aliens have been interacting with humans for thousands of years, and that ancient artifacts and artwork, such as the pyramids in Egypt and the Nazca Lines in Peru, are evidence of this. Others argue that some religious texts, such as the Bible, contain references to extraterrestrial life and that these texts have been deliberately misinterpreted by governments and religious organizations. 

Despite the lack of evidence, many people continue to believe in these theories about aliens and UFOs. However, it is important to note that the scientific consensus is that there is no evidence of current and past extraterrestrial life visiting earth. Many claims made by theorists have been debunked and there are scientific explanations for the phenomena they cite as evidence.  In recent years, there has been a growing interest in the study of astrobiology, the branch of science that deals with the possibility of extraterrestrial life. The search of microbial life or bio-signatures outside of Earth is an active area of research for scientists. But until today, no solid evidence of the existence of extraterrestrial life has been found. 

In summary, the idea that aliens have visited or are currently visiting earth is a popular conspiracy theory, but there is no concrete evidence to support this claim. Many of the claims made by conspiracy theorists have been debunked, and the scientific consensus is that there is no evidence of current or past extraterrestrial life visiting Earth.