From The Mother: The Story of Her Life, Georges Van Vrekhem :

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“On 17 November 1973, the Mother laid down her residual human body while she continued to exist forever in a body consisting of a supramental substance. The supramental substance as worked out by the Mother is a material substance – otherwise the earthly revolution would have no meaning – though it is composed of a substance more refined than the Matter known to us. Generally speaking, the Gross Matter of our Mother the Earth is still in the process of transformation. This process is now in an advanced stage thanks to the avataric Yoga of Sri Aurobindo and the Mother. When gross Matter will have become sufficiently subtle and receptive, i.e. transformed, the mature souls, ready and waiting in their soul-world, will descend and incarnate in it. The formation of their bodies will be moulded by the existence of the Mother’s supramental body, the prototype of the new species.”

Three days after her passing the Mother’s body was placed in the Samadhi along side Sri Aurobindo.

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Read below the narration on The Mother's Samadhi by Satprem and Udar.