Late in his life, in 1995/96, Eduardo Chillida designed a project to create a huge space in the inactive Tindaya volcano on Fuerteventura, in which visitors would be able to experience moonlight or sunlight falling in through two different lightshafts. The shafts would project the incoming light in a geometrical form into the space. Unfortunately, Chillida’s proposal could not be realized. James Turrell’s project for the Roden Crater in Arizona, however, has been built in stages since 1972. This, too, involves an inactive volcano and it is therefore instructive to compare both artists’ visions in these projects. What were Chillida’s plans and why could they not be realized? My own experiences on the Tindaya Mountain in 2018 help me to find some answers.