My stepfather has lost his shit and gone from accountant to flat earther. This has evolved from glass dome above the earth that caused the Challenger explosion to some new shit. Enter the ２０４０ Ｐｈｏｅｎｉｘ Ｅｖｅｎｔ. Every 140 years there's a cataclysmic event which changes the fundamental nature of the planet. This cycle is so powerful that it actually destroyed the Mayan civilisation. In 2040 it's due to happen again and the ONLY place on earth that will be safe is Egypt.
But what was the last thing that happened in 1900, you might ask.
The 1908 Tunguska Event in which an asteroid air burst above Siberia and killed a few reindeer herders.
But 1908 is 8 years after the 140 year cycle and 1900 was pretty uneventful, you might say.
Arguing is the worst thing you could do, they don't believe this stuff because it makes sense, they believe it because it makes them feel special. If you attack it, you're attacking them.
The trick is to get them into healthier hobbies, slowly but surely move their sense of self-worth away from conspiracies and into something that makes them feel genuinely validated in their lives. Conspiracy theories are a symptom, not a cause.