"Zep tepi" has long been a difficult concept to solve by context because it doesn't even appear directly in the Pyramid Texts.

304a. To say: There is a clamour in heaven.
304b. "We see a new thing," say the primordial gods.
304c. O Ennead, a Horus is in the rays of the sun.

This is not even in reference to "zep tepi" which appears to refer to the mythological time of the "first eruption". Perhaps the opening of "2001 A Space Odyssey" with the monkeys fearful of the alien object is a sort of racial memory of zep tepi. In Egyptian times this object still stood on the Giza Plateau.

The clues all seem to support the assumption that this is the meaning but I always lacked any conviction since the evidence is all circumstantial. But I've found an interesting line from the Coffin Texts which seems to cement this in place just a little bit.

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The knot is tied behind me by Set, in whose power the ennead were at first, before uproar had come into being, so that you might make me hale as this one who slew my father, for I am he who takes possesion of the [two] lands.

The knot is tied behind me [by] Nu who saw [the first Occasion] before the gods were born, [...].

Zep tepi is well described in the PT but is not the subject so is not named scientifically.

1039a. To say: Greetings to you, Waters, which were brought by Shu and lifted up by the two sources,
1039b. in which Geb bathed his limbs,
1039c. so that hearts were in the following of fear and hearts were in the following of terror.
1040a. N. was born in Nun,
1040b. when the sky had not yet come into being, when the earth had not yet come into being,
1040c. when the two supports (of the sky) had not yet come into being, when unrest had not yet come into being,
1040d. when fear had not yet come into being, which came into being on account of the eye of Horus.

Shu (upward) brought the water from atum and Iusaas (his "consort") (the hole in the ben ben which also brought the wasdjet). N was born in Nun (water) as surely as atum (the very first god) was pre-existed by Nun. Without water there was no eruption and no birth of upward and downward; there could bew no zep tepi.

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The knot is tied behind me [by] Nu who saw [the first Occasion] before the gods were born, [...].

Nun saw zep tepi through the eye of horus in the land of rainbows.