קבורת חמורים היא זו
This is like a burial of donkeys and is disrespectful.
לר' יוחנן דעביד להו בקרן זוית
The Gemara answers: According to the opinion of Rabbi Yoḥanan, each of these two niches is formed in a different corner of the chamber, aligned diagonally away from the chamber’s cavity.
והא נגעי כוכין להדדי
The Gemara challenges: But if so, won’t the niches located around the corners touch one another? According to Rabbi Yoḥanan’s suggestion, there are, in fact, three niches located at each of the two back corners. One is recessed into the wall of the length of the chamber, at right angles to it, another is recessed into the back wall of the chamber, also at right angles to it, and a third that extends diagonally away from the chamber. The ends of these three niches will overlap, causing there to be too little space in them to fulfill the minimum required measurement.
אמר רב אשי במעמיק דאי לא תימא הכי ארבע מערות לר' שמעון היכי עביד להו הא קא נגעי כוכין להדדי אלא במעמיק הכא נמי במעמיק
Rav Ashi said: It is possible to do this where, when constructing the niches that extend diagonally away from the chamber, one deepens them, i.e., forms them lower down in the wall than the other niches adjacent to them. As, if you do not say that this is a solution, then how could the four chambers depicted by Rabbi Shimon be constructed? Won’t the niches touch one another? The niches of one chamber will overlap those of the chamber adjacent to it. Rather, it is possible arrange the niches only where, when constructing the niches of two chambers at opposite ends of the courtyard, one deepens them, i.e., forms them lower down in the wall than the niches of the other two chambers that are adjacent to them. Here too, it is a case in which one deepens them.
רב הונא בריה דרב יהושע אמר ארבע מערות לר"ש דעביד להו כי חרותא
Rav Huna, son of Rav Yehoshua, said that there is an alternative explanation: It is possible to fashion the four chambers depicted by Rabbi Shimon where one constructs them so that all the niches extend diagonally away from the cavity of the chamber like the leaves of a palm branch, which have spread out from its spine. If formed in this way, the niches of adjacent chambers will not overlap, even if they are at the same depth in the ground.
והא דרב הונא בריה דרב יהושע ברותא היא מכדי כל אמתא בריבועא אמתא ותרי חומשי באלכסונא כמה הוי להו חד סרי וחומשא כוכין כמה הוו תמניא תמניא בחד סרי וחומשא היכי משכחת לה אלא הא דרב הונא בריה דרב יהושע ברותא היא
The Gemara notes: And this statement of Rav Huna, son of Rav Yehoshua, is an error, because it is not possible to form all the niches in this way, since the length of the diagonal of every square whose side is one cubit is one and two-fifths cubits. Each chamber is eight cubits long, which means that between two adjacent chambers, placed at right angles to each other, there is a square area of eight cubits by eight cubits into which the niches of those two chambers must be formed. How much is the diagonal of that area? Eleven and one-fifth cubits. How many niches have to fit in that space? Eight. How can you find that eight niches can possibly fit into eleven and one-fifth cubits? Each of the niches is a cubit wide, and there must be a cubit wall of bedrock between each niche. To accommodate this once would need an area of at least fifteen cubits. Rather, this statement of Rav Huna, son of Rav Yehoshua, is in error.
ואי בעית אימא כדאמר רב שישא בריה דרב אידי בניפלי הכא נמי בניפלי
The Gemara provides an alternative solution to the problem of the niches at the corners overlapping. Or if you wish, say as Rav Sheisha, son of Rav Idi, said in answer to a similar question: It is referring to non-viable newborns, which require only very small burial niches. Similarly, here also, the niches at the corners of the chambers were used for non-viable newborns.
תנן התם המוצא מת מושכב כדרכו נוטלו ואת תפוסתו שנים נוטלן ואת תפוסתן
§ The Gemara considers another mishna that relates to the structure of a catacomb: We learned in a mishna there (Nazir 64b): With regard to one who finds a corpse while inspecting a place that was not previously established as a cemetery, to ensure that it is free of ritual impurity, and the corpse is lying in the usual manner of Jewish burial, he removes it from there and also removes its surrounding earth. And similarly, if he finds two corpses buried in the usual manner, he removes them and their surrounding earth. Since only one or two corpses were found, it may be assumed that the area is not a cemetery; therefore, the corpses may be removed.