Solaris is a science fiction novel by Stanislaw Lem, published in Warsaw in 1961. It was adapted into to a film in 1972 and again in 2002. See Solaris (movie).

Warning: Wikipedia contains spoilers.

The novel is about a scientific investigation on a planet with a strange ocean, which turns out to be, possibily, alive. Lem uses this as a device to dig deeply into human psychology.