The Merapi Terraforming Project is an art-science project on the Merapi volcano in Indonesia, set up in collaboration with the artist collective House Of Natural Fiber (HONF). Merapi is one of the most active volcanoes in the world, and erupts on regular intervals every few years. The year 2010 saw the last series of intense eruptions which covered a large area with lava and volcanic ash, with an estimated 300,000 people being displaced. Contrary to popular belief, volcanic material of Merapi is not a good substrate for growing plants since it does not contain nitrogen, one of the key elements for plant metabolism. The Merapi Terraforming Project consists of a biological art work that is erected on the flanks of the volcano. In collaboration with Gadjah Mada University (UGM), nitrogen-fixing Rhizobia bacteria are used to grow legumes as pioneering plants in volcanic soil. The bacteria trap atmospheric nitrogen and pass it on to the plants. The plants are cultivated in an architectural structure that operates both as monument and vertical laboratory. The project is explicitly inspired by the concept of terraforming, and draws upon the imaginary of colonization barren worlds in outer space. The art work is developed in close collaboration with artists from Yogyakarta, biology and architecture students from UGM, and the local population from the Merapi region. The project was started in July 2011, and local collaborators ran their own small-scale experiments, used mobile phones to document the project, and social networks to disseminate updates. Some plants were grown successfully, others perished due to drought or heavy rainfall washing away vegetation. The project generated considerable debate among the local population, and became the subject of heated discussions on the future use of destroyed land. The Merapi Terraforming Project illustrates how art, astrobiology and social outreach can be combined to address real-world issues on Earth.