Are flat-Earthers being serious?

Members of the Flat Earth Society claim to believe the Earth is flat. Walking around on the planet’s surface, it looks and feels flat, so they deem all evidence to the contrary, such as satellite photos of Earth as a sphere, to be fabrications of a “round Earth conspiracy” orchestrated by NASA and other government agencies.

The belief that the Earth is flat has been described as the ultimate conspiracy theory. According to the Flat Earth Society’s leadership, its ranks have grown by 200 people (mostly Americans and Britons) per year since 2009. Judging by the exhaustive effort flat-earthers have invested in fleshing out the theory on their website, as well as the staunch defenses of their views they offer in media interviews and on Twitter, it would seem that these people genuinely believe the Earth is flat.

But in the 21st century, can they be serious? And if so, how is this psychologically possible?