I saw the film at the end of the 90’s. in Germany on TV. It was in black and white. There is a group of friends, they go to a cave from time to time. Inside they get rid of their clothes except for the underwear. It appears that they cannot travel through a hole in the cave when they have their clothes on.
Through the hole, they get into some kind of future. From visit to visit they expand their area inside the future.
The future itself is pretty empty, so no other humans.
I dont know what happens next exactly. The guys get some kind of radiation illness and some cant travel back, so they travel around without a clue.
At the end the last one dies, one long camera shot on a street with the dead guy. A car arrives, a man puts the body into the trunk and drives off. Inside the car a young boy ask his father something like this:
Dad, how will our car drive if we dont find dead people anymore?