From the days of Sarah Imeinu (matriarch), Jewish women have immersed in mikveh monthly. At the time of the Temple, this was also very common by men because of the laws of purity that were then in practice. And until today, many religious Jews go to mikveh at least before Shabbos, and many, especially the chasidic, go daily. Also, until today, an eating utensil purchased from non-Jews is to be dipped in a cosher mikveh before use. Here are just a few amazing stories which emphasize the reality of purity.
The kaballists said that all one's deeds are written on his face. The holy Ha'Ari, Rebbe Yitzhak Luria, would tell people if he saw on their countenace a transgression of Jewish law, which had left its mark. Many later Tzadikkim were capable of the same power. One was Rebbe Mordechai of Chernobil. He said, when a person comes to me, I see everything he has done even in his innermost chambers. But, he added, if he goes first to mikveh, then I do not see anything bad. His grandson was Rebbe Duvid of Zlatapole. Once one of his chasidim passed through his town Friday. Being in a rush, he thought to go in to the Rebbe without going first in mikveh, as was customary by him. He just got close to door of the Rebbe, and without even seeing him, the Rebbe said to his shammesh (attendant), ask him why he doesn't go first in mikveh?
The holy Belzer Rebbe Reb Aharon, was known not to shake the hand of anyone who came to him, unless they had been that day in mikveh. There are many stories from people still with us today how they had seen that he would know this, and when he sensed that the person had not been, he would wrap his hand in a towel and only that way touch him. Once, in Belz in Europe, his gartel (ceremonial belt) fell off while walking through the shul. A young Torah student lunged on it and gave it to him, rejoicing in the opportunity to serve the great Rebbe. But the Rebbe said to him, "who asked you, who told you? Were you in mikveh today?" So holy was he. And once, in Tel Aviv when he was older and did not go frequently, he sent to ask the mikveh attendant if he has a completely fresh mikveh, that no one had been in (not for cleanliness reasons, rather for spiritual). The attendant answered affirmatively, and so the Rebbe went there and "toivelled" (immersed in Yiddish). Later he sent to call the mikveh attendant. The Rebbe told him, you don't have to worry about me. If you don't have a fresh one, you don't. That was because one person had been in it before, though either the attendant forgot or didn't think it important enought to mention it. But the Tzadik felt it
There are many stories about Tzadikim who recognized on an eating vessel if it had been immersed in mikveh or not (besides many other stories about recognizing if food was not completely cosher). One of them, Rebbe Arie Leib Tzinz, when asked how did he know, answered that on a vessel that has been properly immersed, he sees the name of G'd (the four Hebrew letters as it is written but strictly forbidden to say) on it. And if he doesn't see that, he knows it has not yet been dipped.
Women's obigation to go to mikveh is extremely great in Jewish law, not like men's custom which is only "optional". There were women who had no mikveh in their city, just a river (which is not always considered a mikveh-a Rabbi must be consulted). In the winter, they would cut a hole in the ice to immerse. And they were rewarded with brilliant children. The mother of the tzadik Reb Hershel Lisker lived in a small village which had no mikveh. But she was so G'd fearing, that Hashem helped her, and when the time came each month, the wind would blow fiercely and dig a hole, and then rain would come and fill it up, and that was her mikveh. (Again the laws of mikveh are myriad and only a Rabbi can decide if it meets all requirements)