Sekine completed the Graduate Program at Tama Art University in 1968. He studied under Yoshishige Saito and created works questioning the relation between the visual sensation and cognition. Phase—Mother Earth, which he submitted to the Contemporary Sculpture Exhibition at the Suma Detached Palace Garden, Kobe in 1968, is highly acclaimed as a representative example of postwar Japanese art. After participating in the Venice Biennale in 1970, Sekine stayed in Europe until the end of 1971. After his return to Japan, in 1973, he founded Environment Art Studio Inc.
In 1968, Sekine presented Phase—Mother Earth, a work in which a cylindrical hole was dug in the ground and the soil dug up was heaped in the same shape. It was completed on a scale of 2.7 meters in height and 2.2 meters in diameter. Its ideological slant linked to the Oriental philosophy that the amount of mother earth does not change even if it is uneven and the vivid materialistic presence of the soil had an enormous impact on the trends of the time, which were reviewing art from its basis, and is still widely known today through photographs in Japan and abroad. This print is composed of a photograph of Phase—Mother Earth, drawings of the plan, and photographs of the work in progress. The drawings with notes reveal that this work was planned as the process of “an experiment on thought,” in which, by digging a hole in the earth and heaping the soil that was dug up, it would become “an inverted earth (i.e. proof of the anti-earth).”