Probably read 15-20 years ago. English Paperback.
Scientists running a deep underground Neutrino Detector [might have been cosmic rays]. It was a VERY Large tank of water with detectors built into the tank walls looking for (flashes?) as the particles interacted with the water as they passed through (I think it happened Very Rarely)
Suddenly alarms go off, the detectors are going crazy and the tank is over-pressured. A Tech who happened to be down doing maintenence at the tank, (The control room is on the surface, and the tank is like a mile underground) opens an access hatch to the tank and discovers a man in the tank who is alive.
The story is now told in alternating points of view: People in This world, and People in the world of his origin. In his world the water tank had been designed for something else. [Maybe a super computer, or Super collider or some other advanced research tech involving super conducting magnets?]
The scientist at the facility in this world wants to know, who he is, how he got in the tank, (which is now contaminated, will need to be emptied, cleaned, repair the detectors, and refilled with (heavy water? ultra pure water?) IIRC, one of the administrators was ranting and raving that the little prank someone pulled was going to cost millions to fix) end eventually, just What is he?
The other world are focused on, what happened, where is he, etc. But I think it was more focused on his family who were the protagonist of the story. (Maybe it was a private experiment who his family were running?) I seem to recall it was a female family member (wife? sister? mother?) who was very involved in the search for answers.
The crux of the story is he was not quite human. I forget the specifics. Maybe he was telepathic. He is close to human, But the difference is enough I think to convince our world, that he must be from an alternate world. And they are trying to figure out how it happened. The man has no idea what happened. One second he was in his world in the underground facility, and next he is drowning in a huge tank of water.
I also have a vague recollection of his world it might have been a matriarchy society. I do seem to recall a separation of the sexes. Not mixing unless married?