TT #214 · Parshas Shemini · Aliyah 7 · Passuk 36
Chabad.org — Vayikra Chapter 11 with Rashi
English: “A spring and a pit”: just as the essential source of a spring is in the ground, so the essential source of a pit is in the ground.
Toras Kohanim.
Sefaria — Sifra (Toras Kohanim), Shemini
Analogy — The Chazal derives the stated rule from the wording and structure of the verse.
The relevant pit is fundamentally grounded.