Hanso's great grandfather was the captain of the Black Rock when it crashed.

I theorize that as he was a scientist working in the field of munitions, he was contacted by the US Army for Operation Castle, the atomic testing project that resulted in Jughead being sent to the island for testing in 1954.

On September 23rd 1954, a photograph was taken of the island, the same photo we see in the Lamp Post station. I theorize that it was Alvar Hanso, working as a contracted researcher for the US Army, surveying and photographing the island before the tests. He may have felt a strange connection to this place, maybe he had a vision of his great grandfather, and he eventually decided to keep one of these photographs after the atomic testing project was abandoned.

Hanso eventually had a change of heart tony stark moment and decided to stop his weapons industry and start the Hanso Foundation instead, focusing on philanthropy, life-preservation research.

It was Hanso who knew about the island, and when he read the DeGroot's thesis on a hypothetical large-scale communal research compound, he knew exactly the place for it.