This chapter examines planetary engineering and human-made biospheres from the perspective of the concept of (un)naturalness using terraformed Mars as a case study. It has been suggested that in the future it may be possible to make Mars habitable for life from Earth. This hypothetical process is known as terraforming or planetary ecosynthesis. The possibility of establishing a biosphere on Mars, or some other celestial body, opens up an interesting case of a biosphere that is unnatural. Furthermore, the concept of natural is central to many arguments in environmental ethics. For example, it has been suggested that the value of an ecosystem is dependent upon its history – it being natural of origin. Naturalness and unnaturalness discussions have centered around two questions. First, is naturalness morally relevant or should it otherwise affect decision-making? Second, how can naturalness be understood in different cases? This chapter addresses both questions in relation to terraforming Mars. First, it analyzes in what sense terraformed Mars and its biosphere would be unnatural. Second, it examines whether its unnaturalness undermines the value of the biosphere, and thus gives an important criterion for deciding whether humans should aim to terraform extraterrestrial environments or not.