There are too many people who have encountered UFOs who are not only are highly credible, but also the UFO was too close to have been mistaken from swamp gas or a weather balloon.
There is also PHYSICAL evidence that can be tested scientifically of alien encounters that when a sceptic finds an answer, he sounds more like a babbling lunar landing conspiracy theorist.
Take for example the Betty Cash saga where Betty Cash, Vicki Landrom and her grandson Colby encountered a UFO in late December of 1980. They saw a craft that had a bright beam at the bottom of it where Betty got out to get a closer look and ran back to her car where Vicki and Colby were.
Vicki put her hand on the dashboard and an impression of her hand was left on it as if it melted. And you need about 200*F for that to happen.
After the incident all 3 started to suffer ill side effects from the encounter which included nausea, vomiting, redness of the skin, hair loss, and cancer with Betty suffering the worst because she was closer to the UFO. And if you Google the symptoms the first thing that comes up is the side effects of chemotherapy and radiation therapy. Even WebMD has radiation poisoning as a possible cause for the listed symptoms.
What do sceptics say she saw? The star Canapes. Or even a comet. Yet how do they explain their symptoms that only started a very short time after their encounter or whatever that was?
Another typical finding is where a UFO landed in a field, and the next day there is a burn mark on the field where the soil directly below is sterilized several feet down, and I mean STRAIGHT down. Even science says that the soil was exposed to high levels of radiation. Sooooo -- do sceptics say it was radioactive swamp gas that using a magic wand made sure it only stayed straight down?