Eruvin 34B

פשיטא! רשות היחיד עולה עד לרקיע, וכי היכי דסלקא לעיל, הכי נמי דנחתא לתחת. ואלא דקאי ברשות הרבים.

it is obvious, for the private domain ascends to the sky, and just as it ascends upward, so too, it descends downward to the bottom of the pit, even if it is more than 10 handbreadths deep. Rather, we must say that the pit is situated in the public domain.

דנתכוון לשבות היכא? אי למעלה ⁠— הוא במקום אחד ועירובו במקום אחר הוא. אי למטה, פשיטא ⁠— הוא ועירובו במקום אחד!

The Talmud now clarifies: Where did one intend to establish his Shabbat residence? If he intended to establish his residence above the pit in the public domain, this is a case where he is in one place and his eiruv is in another place, i.e., in a private domain, and therefore his eiruv is not valid. Alternatively, if one intended to establish his Shabbat residence below, in the pit, it is also obvious, as he and his eiruv are in one place.

לא צריכא, דקאי בכרמלית ונתכוון לשבות למעלה, ורבי היא דאמר: כל דבר שהוא משום שבות לא גזרו עליו בין השמשות.

The Talmud answers: This ruling is necessary only in a case where the pit is situated in a karmelit, and he intended to establish his Shabbat residence above the pit in the karmelit. And with regard to the question of how this eiruv can be valid, as one cannot bring the eiruv from the pit to the karmelit, the answer is that the Mishnah was taught in accordance with the opinion of R' Yehuda HaNasi, who said: With regard to anything that is prohibited on Shabbat due to rabbinic decree [shevut], they did not issue the decree to apply during twilight. Since carrying from the pit to the karmelit is only prohibited as a shevut, a person may carry from the pit to the karmelit during twilight, the time when the eiruv establishes one’s Shabbat residence.