Back in the day I had to read a story from the "recommended" books section and I chose this.
The story is about a guy (let's call him Steve) having to visit a planet that already has a human base set up, but I think they lost contact with the inhabitants, which prompted the reason for Steve to be sent off there. He had some kind of experience that was useful for the mission. As I remember, Steve wasn't that keen on the idea because the planet was pretty far, so it would take years to reach it.
When Steve reached the planet, it turned out that most of the inhabitants were gone sans for one guy he finds later at the base.
The planet had a vast ocean that, the longer Steve was on this planet, the more he felt like it could talk to it. Something along the lines of the ocean being a sentient being.
After being on the planet for a while, Steve suddenly finds his girlfriend there, which should be impossible. Pretty sure he found her naked, and by close inspection it turned out that she had smooth soles "smooth as a child's that has never walked before", implying it's a clone planted by the sentient ocean for X reason. He later develops feelings for this girl (since she looks like his girlfriend/wife). I also think that the original girl is dead, or they broke up, some tragic background story. The girl was worried that Steve likes her because she looks like his previous partner, but Steve assures her that while they look the same, she acts different.
Steve got a cell sample from the girl, and upon closer inspection under the microscope it turned out that the cells are made up of something smaller than an atom, so small that their equipment couldn't focus enough to actually see the thing the cell is made of.
I never finished the story, so I don't know what happens in the end, but that's all the info I remember.