Mikvah is a specially constructed pool of water used for total immersion in a purification ceremony within Judaism. Its main uses nowadays are by Jewish women to achieve ritual purity after menstruation or childbirth, by Jewish men to achieve ritual purity (see details below), as part of a traditional procedure for conversion to Judaism, and for utensils used for food.

A mikvah is central to an Orthodox Jewish community. A Jewish community is traditionally required to build a mikvah (for women) before they build a synagogue, and to sell a Torah scroll to build one if need be. 

HISTORY 

Traditionally, the mikvah was used by both men and women for various purposes. Everyone was required to go to the mikvah upon coming into contact with the dead or other ritually unclean: 

Sanctifing the people: (Ex:19:10: And the LORD said unto Moses, Go unto the people, and sanctify them to day and to morrow, and let them wash their clothes, Ex:19:14: And Moses went down from the mount unto the people, and sanctified the people; and they washed their clothes. 

Entering into the House of Yahweh: (Ex:29:4: And Aaron and his sons thou shalt bring unto the door of the tabernacle of the congregation, and shalt wash them with water. Ex:30:18: Thou shalt also make a laver of brass, and his foot also of brass, to wash withal: and thou shalt put it between the tabernacle of the congregation and the altar, and thou shalt put water therein. Ex:30:19: For Aaron and his sons shall wash their hands and their feet thereat: Ex:30:20: When they go into the tabernacle of the congregation, they shall wash with water, that they die not; or when they come near to the altar to minister, to burn offering made by fire unto the LORD: Ex:30:21: So they shall wash their hands and their feet, that they die not: and it shall be a statute for ever to them, even to him and to his seed throughout their generations. Ex:40:30: And he set the laver between the tent of the congregation and the altar, and put water there, to wash withal.) 

Eating unclean animals: (Le:11:28: And he that beareth the carcase of them shall wash his clothes, and be unclean until the even: they are unclean unto you. Le:11:40: And he that eateth of the carcase of it shall wash his clothes, and be unclean until the even: he also that beareth the carcase of it shall wash his clothes, and be unclean until the even.) 

Plagues: (Le:13:6: And the priest shall look on him again the seventh day: and, behold, if the plague be somewhat dark, and the plague spread not in the skin, the priest shall pronounce him clean: it is but a scab: and he shall wash his clothes, and be clean.) 

Man or woman issues: (Le:15:5: And whosoever toucheth his bed shall wash his clothes, and bathe himself in water, and be unclean until the even. Le:15:6: And he that sitteth on any thing whereon he sat that hath the issue shall wash his clothes, and bathe himself in water, and be unclean until the even. Le:15:7: And he that toucheth the flesh of him that hath the issue shall wash his clothes, and bathe himself in water, and be unclean until the even. Le:15:8: And if he that hath the issue spit upon him that is clean; then he shall wash his clothes, and bathe himself in water, and be unclean until the even. Le:15:25: And if a woman have an issue of her blood many days out of the time of her separation, or if it run beyond the time of her separation; all the days of the issue of her uncleanness shall be as the days of her separation: she shall be unclean. Le:15:26: Every bed whereon she lieth all the days of her issue shall be unto her as the bed of her separation: and whatsoever she sitteth upon shall be unclean, as the uncleanness of her separation. Le:15:27: And whosoever toucheth those things shall be unclean, and shall wash his clothes, and bathe himself in water, and be unclean until the even.)  

Sex or wet-dreams: (Le:15:16: And if any man's seed of copulation go out from him, then he shall wash all his flesh in water, and be unclean until the even. Le:15:17: And every garment, and every skin, whereon is the seed of copulation, shall be washed with water, and be unclean until the even. Le:15:18: The woman also with whom man shall lie with seed of copulation, they shall both bathe themselves in water, and be unclean until the even.) 

For eating things that died on their own: (Le:17:15: And every soul that eateth that which died of itself, or that which was torn with beasts, whether it be one of your own country, or a stranger, he shall both wash his clothes, and bathe himself in water, and be unclean until the even: then shall he be clean. Le:17:16: But if he wash them not, nor bathe his flesh; then he shall bear his iniquity.)  

Requirements of a Mikvah 

The rules regarding the construction of a mikvah are complicated. The immersion itself must take place in a pool which is: a "ma'ayan" (spring), or a well of naturally occuring water, containing a minimum quatity of water, or connected in halachically prescribed manner to one of the following: Rainwater, certain springs, wells, rivers and lakes can therefore be used for the pool (Ex:2:5: And the daughter of Pharaoh came down to wash herself at the river; and her maidens walked along by the river's side; and when she saw the ark among the flags, she sent her maid to fetch it.), as can snow (Job:9:30: If I wash myself with snow water, and make my hands never so clean;) and other naturally precipitated frozen water. There are also requirements for the manner in which the water can be stored and transported to a pool. In general, water must flow by gravity (it cannot be pumped or carried). As a result, tap water cannot be used for the "natural water" component of a mikvah, and a special construction is required. Most contemporary mikvas are indoor constructions involving rain water collected in a cistern (bor) with a duct to a regular bathing pool so that it can flow by gravity into the basin. Once the minimum required amount of "natural water" is added, regular tap water can be used to augment the amount, which has a connection to the pool of rain water. This later formula is often used so that the water used for immersion can be frequently changed. The duct can be closed to empty and replace the regular water without having to replace the rain water. A mikvah must contain a minimum of forty se'ah of water, approximately 200 gallons or 750 liters. If certain precautions are observed, the water in a mikvah can be heated. The environment of a contemporary mikvah is often not unlike a spa.  

Our mikvah here is supplied by a reasonable amounts of naturally precipitated morning dew in the spring and summer (due to the fact that it placed on the side of a mountain peek where over head clouds descend into the below valley), rainwater in the fall and winter times by a natural form of irrigated runoff from the slopes of the hills (and also overhead rainfall). We also have occassional snow in the winter that provides the minimum amount of natural water, which we added tap water to meet the necessary amount.