I recall reading this book, maybe ten years ago and LOVING it, but can't remember the name, the author, or even when I read it. I remember vague details — but the details are foggy — so I may have some of it wrong.
Some people (possibly scientists) get lost in the deep ocean in a submarine. Somehow, after going on deep down, they emerge in a futuristic room, and after entering a door they end up in a new world.
There are human-like beings, friendly and welcoming who are pleased to show the submarine group how they live.
One thing that stands out in my memory is the beings have sex by pressing the palms of their hands together and it supposedly feels insanely more intense than our sex, when the sub group try it with the women from the new world.
Even though the beings are accommodating, have parties in the sub group's honor, and welcome them (some members actually falling in love with one of the beings), over time a couple of the sub group people decide things feel a little off and they'd like to go back to their lives.
They get back in their submarine and go back through whatever portal brought them there. Eventually they emerge on the surface of the ocean, and in time spot a boat. But it looks like an old fashioned ship. Somehow they determine they have come up in the 1800's rather than the time they were from.
I Googled for quite a while and couldn't figure it out for the life of me. But I'd really like to read it again. If you can help that'd be great! Thanks!