Mishnah Mikvaos 7:7: If one immerses a bed in it [in a mikveh containing precisely forty se'ah], even if its legs sink into thick mud [at the bottom of the mikveh, which is not counted as part of its waters] it is pure, because the waters precede it. If the waters of a mikveh are shallow [such that one cannot immerse one's whole body at once], one may weigh down even bundles of wood or bundles of reeds [with weights, on one side of the mikveh,] so that the waters will rise, and then he can descend and immerse. Regarding a [impure] needle on the steps of a cavern [which houses a mikveh], if one was moving the waters back and forth [by stirring the waters around with one's hands or feet], once a wave passes over it [the needle], it is pure.

הִטְבִּיל בּוֹ אֶת הַמִּטָּה, אַף עַל פִּי שֶׁרַגְלֶיהָ שׁוֹקְעוֹת בַּטִּיט הֶעָבֶה, טְהוֹרָה, מִפְּנֵי שֶׁהַמַּיִם מְקַדְּמִין. מִקְוֶה שֶׁמֵּימָיו מְרֻדָּדִין, כּוֹבֵשׁ אֲפִלּוּ חֲבִילֵי עֵצִים, אֲפִלּוּ חֲבִילֵי קָנִים, כְּדֵי שֶׁיִּתְפְּחוּ הַמַּיִם, וְיוֹרֵד וְטוֹבֵל. מַחַט שֶׁהִיא נְתוּנָה עַל מַעֲלוֹת הַמְּעָרָה, הָיָה מוֹלִיךְ וּמֵבִיא בַמַּיִם, כֵּיוָן שֶׁעָבַר עָלֶיהָ הַגַּל, טְהוֹרָה:

Mishnah Mikvaos 8:1: The land of Israel is pure and her mikvaot [gatherings of water collected by natural means and immersed in for purification] are [assumed to be] pure. The mikvaot of the nations outside the Land are valid for ba'alei kerayin [males who have had a seminal emission but have yet to purify themselves by immersion in a mikveh], even if they were filled manually from a pit of water. The ones in Israel that are outside the locks [of the city gates, i.e. outside the cities] are valid even for niddot [women who have menstruated and are thereby impure]; those inside the locks [i.e. inside the city gates], are valid for ba'alei kerayin but invalid for all [other] impure individuals. Rabbi Eliezer says: [even outside the city gates,] those close to the city and to the road are impure due to being used for laundry, and those far away are pure.

אֶרֶץ יִשְׂרָאֵל טְהוֹרָה, וּמִקְוְאוֹתֶיהָ טְהוֹרִים. מִקְוְאוֹת הָעַמִּים שֶׁבְּחוּצָה לָאָרֶץ, כְּשֵׁרִים לְבַעֲלֵי קְרָיִין, אֲפִלּוּ נִתְמַלְּאוּ בְקִילוֹן. שֶׁבְּאֶרֶץ יִשְׂרָאֵל, שֶׁחוּץ לַמַּפְתֵּחַ, כְּשֵׁרִים אַף לְנִדּוֹת. מִלִּפְנִים מִן הַמַּפְתֵּחַ, כְּשֵׁרִים לְבַעֲלֵי קְרָיִין, וּפְסוּלִים לְכָל הַטְּמֵאִים. רַבִּי אֱלִיעֶזֶר אוֹמֵר, הַקְּרוֹבִים לָעִיר וְלַדֶּרֶךְ, טְמֵאִים, מִפְּנֵי הַכְּבִיסָה. וְהָרְחוֹקִים, טְהוֹרִים: