So from what I understand, me'arah (Artscroll translates as vault) is a rectangular prism air pocket dug out underground with a small tunnel leading to the surface, if this is correct, would the 10 tefachim be the bottom point of the "box" or the point of contact of the animal with the tunnel, like for example let's say the tunnel goes down then curves into the vault, would the 10 tefachim count just before the curve and the animal slides into the vault and dies by air, or the tunnel is shorter and the animal slides in and dies while on the 10 tefachim from the surface floor? This is according to Rav, Shmuel would hold liable because of impact
Avraham Katz, New York City, New York, United States
My initial reaction is that there must be 10 tefachim of the me'arah without the small tunnel. This is because according to Rav one needs 10 tefachim of air to kill. I should have thought that the bad air in the tunnel does not combine to make the me'arah lethal. Similarly, according to Shmuel one needs a clean fall of 10 to give enough impact to kill. The tunnel would not be counted to make up that 10 tefachim.
Thinking about this again, I do not think that it is correct to say that the me'arah possesses a tunnel. Rashi writes that the "me'arah is square and covered over with a ceiling, but it has a 'mouth'". I do not see why we should say that this "mouth" is a tunnel. I understand that "mouth" simply means an opening. The me'arah is a square underground hollow with a ceiling but there has to be an opening where an animal can fall into. That is why Rashi writes that it possesses a mouth.
Now the question of where the 10 tefachim start from is answered. They start at ground level at the top since there are 10 complete tefachim between ground level and the surface of the me'arah at the bottom.
A proof that the me'arah does not possess a tunnel:-
Rashi writes that the me'arah has a Peh and in fact the phrase ?? ????? "the mouth of the cave" is quite a common one in Chazal. I think it would be translated as "the opening of the cave" but one would not say it means a small tunnel leading to the surface of the cave.
We find the phrase "Pi Hame'arah below in Bava Kama 117b where Rashi, on the top line of the page, writes that a large snake made himself into a circle and surrounded Pi Hame'arah. This means that the snake blocked up the opening of the cave.
I think that the word that would be used for a small tunnel would probably be "Tzinor"; a pipe.
Dovid Bloom