Sri Aurobindo translates Taittiriya Upanishad, Shikshavalli, Chapter 5: “Bhur, Bhuvar and Suvar, these are the three Words of His naming. Verily the Rishi Mahachamasya made known a fourth to these, which is Mahas. It is Brahman, it is the Self, and the other gods are his members. Bhur, it is this world; Bhuvar, it is the sky; Suvar, it is the other world: but Mahas is the Sun. By the Sun all these worlds increase and prosper. … These are the four and they are fourfold; — four Words of His naming and each is four again. He who knoweth these knoweth the Eternal, and to him all the Gods carry the offering.”
Additional descriptions are related in this chapter, relating to the Shining Fires, the Vedas and the breaths. The treatment is similar. Sri Aurobindo describes these three worlds as relating to planes of consciousness. Bhur is the world of the plane of Matter. Bhuvar is the world of the life-plane. Suvar is the world of the mental plane. In this configuration Mahas represents the Supramental plane. The etymology of the name Mahachamasya provides a further clue. “Maha” means vast or great. “chamasya” is the vessel that holds the Soma, the nectar of the gods, the bliss or Ananda of the Immortals.
Sri M. P. Pandit notes regarding Mahas: “This is the vyahrti which is termed the Supermind by Sri Aurobindo, the Gnosis standing at the summit of this lower creation of the worlds of matter, life and mind. The Upanishad declares that this is Brahman itself, the very Self of the Manifesting Reality, of which the other worlds, the Gods presiding over them, are limbs. They live, increase and prosper by the Power of this Mahas which is compared to the Sun — Mahas is the source of Light (and fire) of the worlds. … Mahas ‘is called the fourth world, because it is above the triple world of ours, bhuh, bhuvah, svah.’ It stands between the two — between the higher triple world of the plenary Sat-Chit-Ananda above on the one hand and the lower triple on the other.It carries within it, above it, and one with it in a sense the higher triple, and it also releases the lower triple world through various agencies for the formation and perfection of the higher half — what is called Parardha — in terms of the lower, the created lower triple. Radically, it is this world of the Mahas that is responsible for the lower triple creation and it is from this world that forces of manifestation of the Many are released and set to play to work out certain possibilities of the Immense Intelligence and power of the Spirit in the finite formations of the world-being and Force-movement. Because its centre is charged with the infinitude of Sat-Chit-Ananda, it is one with it Above; because its face is turned towards Manifestation, it is one with the Many that represent the self-limiting and self-conditioning attitude and capacity of the Supreme Spirit.’ (cited by M P Pandit from T.V. Kapaly Sastry, Lights on the Fundamentals, pg. 42)”