Reference for Tevul Yom 2:8
בִּעְבּוּעַ שֶׁבֶּחָבִית שֶׁנִּקַּב בֵּין מִבִּפְנִים בֵּין מִבַּחוּץ וּבֵין מִלְמַעְלָן בֵּין מִלְּמַטָּן), זֶה כְנֶגֶד זֶה, טָמֵא בְאַב הַטֻּמְאָה וְטָמֵא בְאֹהֶל הַמֵּת. הַפְּנִימִי מִלְּמַטָּה וְהַחִיצוֹן מִלְמַעְלָן, טָמֵא בְאַב הַטֻּמְאָה וְטָמֵא בְאֹהֶל הַמֵּת. הַפְּנִימִי מִלְמַעְלָן וְהַחִיצוֹן מִלְּמַטָּן, טָהוֹר בְּאַב הַטֻּמְאָה וְטָמֵא בְאֹהֶל הַמֵּת:
Regarding a bubble [i.e. a hollow imperfection] in a barrel which was pierced, whether from the inside or the outside, whether from above or below, if [the holes are] opposite one another, it is [susceptible to being rendered] impure from [contact with] an Origin of impurity, and impure from [being enclosed in] a corpse's tent. If the inner [hole] is below, and the outer is above, it is [susceptible to being rendered] impure from an Origin of impurity, and impure from a corpse's tent. If the inner [hole] is above and the outer is below, it is pure regarding an Origin of impurity, but it is [susceptible to being rendered] impure from a corpse's tent.