Eruvin 58B

אבל חוט המשקולת יורד כנגדו ⁠— מודדו מדידה יפה.

However, if a plumb line drops straight down, i.e., if the canyon wall is very steep, he measures the width of the canyon properly at the bottom of the canyon, without taking its walls into account.

וכמה עומקו של גיא? אמר רב יוסף: אלפים.

The Talmud asks: And what is the depth of a canyon that may be spanned if it is not more than 50 cubits wide? Rav Yosef said: Up to 2,000 cubits; but if it is deeper than that, the slope must be measured as well.

איתיביה אביי: עמוק מאה ורוחב חמשים ⁠— מבליעו. ואם לאו ⁠— אין מבליעו? הוא דאמר כאחרים, דתניא, אחרים אומרים: אפילו עמוק אלפים ורוחב חמשים ⁠— מבליעו.

Abaye raised an objection from the following baraita: If a canyon is up to 100 cubits deep and up to 50 cubits wide, one may span it; and if not, one may not span it. How could Rav Yosef say that the canyon may be spanned if its depth is less than 2,000 cubits? The Talmud answers: He stated his opinion in accordance with the opinion of Aḥerim; as it was taught in a baraita: Aḥerim say: Even if the canyon is 2,000 cubits deep and 50 cubits wide, one may span it.

איכא דאמרי, אמר רב יוסף: אפילו יתר מאלפים. כמאן? דלא כתנא קמא ולא כאחרים!

The Talmud cites an alternate version of the previous discussion. Some say that Rav Yosef said: Even if the canyon is more than 2,000 cubits deep, it may be spanned. The Talmud asks: In accordance with whose opinion did Rav Yosef say this? It is not in accordance with the opinion of the first tanna, and it is not in accordance with the opinion of the Aḥerim.

התם ⁠— שאין חוט המשקולת יורד כנגדו. הכא ⁠— בחוט המשקולת יורד כנגדו.

The Talmud answers: There, where the tanna’im disagree about the depth of a canyon that may be spanned, they refer to a case where a plumb line does not drop straight down and therefore there is reason to measure the slope. Here, however, where Rav Yosef says that the canyon may be spanned even if it is more than 2,000 cubits deep, he is referring to a case where a plumb line drops straight down.