I'm looking for a book that I read when I was young (~1985). My guess is that it was published in the 70s or 80s.
In this story, a scientist (or more) created some sort of portal. Only organic tissue could pass through it. Metal (for example dental filling) just dropped on the floor. I remembered a passage where someone was exclaiming that it would be perfect to do medical examination.
After some tests with animals that could go inside and back safely from the portal, someone stuck its head inside, only to find blackness - some sort of cave. They could not touch the bottom, or anything.
They tried to do a human chain to reach the bottom without success (and in fact some people were lost).
A team decided to jump and see what was on the other side. I could not remember what happened next, except that they survived and build a society.
I really tried to identify the book without success. And I would love to re-read it was it is one of these book that I think about regularly.