The Garden of Rama (Rama, #3)

In the year 2130 a mysterious spaceship, Rama, arrived in the solar system. It was huge - big enough to contain a city and a sea - and empty, apparently abandoned. By the time Rama departed for its next, unknown, destination many wonders had been uncovered, but few mysteries solved. Only one thing was clear: everything the enigmatic builders of Rama did, they did in threes. Eighty years later the second alien craft arrived in…