When God created the world, it was all water. As stated in Genesis 1:2 - “Now the earth was unformed and void...and the spirit of G-d hovered over the face of the waters.” On the second day, God divided the waters, setting the “upper waters” as the firmament that formed the heavens. On the third day, God brought forth the dry land and gathered the “lower waters” into what became oceans.
The waters remain bound to each other, however, through the perpetual cycle of evaporation and precipitation. Water thus maintains the connection of the upper and lower realms, and this intimate connection with both heaven and earth give water the power of “purification.” This is reflected in the power of the mikveh, the ritual bath, to cleanse.
A mikveh is not simply a pool of water. In order to be “kosher,” a mikvah must contain 40 se'ah (which in common practice is generally assumed to be not less than 264 gallons) of rainwater gathered naturally in accordance with Jewish law. This rainwater must be stored in a natural container, that is, in essence, part of the ground. In other words, the rainwater from above must mingle with the earth below!
In Temple times, ritual purity, achieved by immersion in the mikveh, was necessary to participate in most Temple rituals. Today, Jewish women use the mikveh to fulfill the laws of family purity, and Jewish men avail themselves of the mikveh voluntarily as a symbol of purity.
Immersing one’s self in the mikveh is not about cleanliness. Before immersing, one must make certain to remove all chatzitzot, barriers between the water and one’s body, such as dirt, band-aids or jewelry (as examples). When completely immersed, a person is enveloped in water that is part of both heaven and earth, which serves as a means to be spiritually reborn.