Now, however, there has been a revival of the ritual, those leaders say, and mikvahs are being built that are decidedly more impressive than the traditional baths familiar to many Orthodox. With elegant suites adorned with exquisite marble tiling and smooth mahogany doors, they have the look of a fancy spa and can be found on the Upper East Side and in the Hamptons, Brooklyn and New Jersey.
"Women come in and say, 'This isn't like any mikvah I've seen before,' " said Shternie Raskin, 35, who with her husband, Rabbi Aaron L. Raskin, 38, runs a well-appointed, renovated mikvah in a synagogue on Remsen Street in Brooklyn Heights that seeks to attract the successful professional women who might otherwise avoid a mikvah.
Rabbi Raskin said: "We wanted to make it like a spa, because you need to make a mikvah much more fashionable if you want to attract career women who are used to upscale resorts. You need to make it like a haven or a utopia."
In Easthampton, a group of investors have bought a house and are building a mikvah in a renovated pool house on the property that they hope to open by late June.
On the Upper East Side, a mikvah is the cornerstone of a $12 million project to turn a town house on East 77th Street into the Schneerson Center for Jewish Life, a 17,200-square-foot space devoted to educational, social and outreach programs. It is set to open in November. "The water drops straight from heaven down into here," said Rabbi Ben Tzion Krasnianski, the director of the center, as he stood recently in the basement where two mikvah pools sunken into the concrete floor resemble miniature swimming pools.
He explained that rainwater was collected on the roof, dripping down through a large pipe and into the mikvah.
A mikvah has two pools: an immersion tank often filled with filtered tap water connected to an adjoining pool that must hold at least 200 gallons of accumulated rainwater and filled according to specific rules. The two pools share a common wall that must have a hole at least two inches in diameter so the waters can touch, or "kiss" as it is sometimes called.
The mikvah in the Jewish center will take up the entire basement with two pools and eight preparation rooms, including a bridal suite to be used by women who want to use the mikvah before their weddings. On the first floor will be a vessel mikvah to purify dishes before use.
"A lot of Jewish people have a negative impression of mikvah," Rabbi Krasnianski said, "but this will be a first-class mikvah, a spa for the soul."
Rabbi Krasnianski's wife, Chanie, said that many non-Orthodox women in her Torah class have begun using mikvahs. One of them, Joyce Misrahi, 36, lives on the Upper East Side with her husband, a hedge fund manager, and their three sons.
"It's unbelievable there isn't a mikvah on the Upper East Side already," Mrs. Misrahi said. "When this new one opens here, I'll go regularly. Having it nice and clean and new makes a big difference."
The mikvah, Rabbi Krasnianski explained, dates to the beginning of Jewish history, and religious law dictates that building one takes precedence over building a synagogue.
Mikvahs are part of the rules governing marital relations among the Orthodox. A husband and wife must refrain from sexual relations from the beginning of the wife's menstrual cycle until after she immerses herself in the mikvah at nightfall, seven days after her cycle finishes. Until then a woman is considered ritually unclean.
The immersion is considered a spiritually purifying ritual, not a hygienic washing, Rabbi Krasnianski said, adding that a woman must be scrupulously clean before she enters the pool. Some Hasidic men use the mikvah before morning prayers, and it is also used for conversions and before Yom Kippur.
Rabbi Zalman Shmotkin, a spokesman for the Lubavitch movement and the director of Chabad.org, a Web site that compiles lists of hundreds of mikvah locations worldwide, said the use of mikvahs declined over the past century because of "wars, upheavals, poverty and less strict forms of Judaism."
In New York City, many mikvahs on the Lower East Side and in other Jewish neighborhoods were steadily shut down, though they are still common in Orthodox neighborhoods in Brooklyn.
The effort to revive mikvahs has been largely motivated by a mandate from Rebbe Menachem M. Schneerson, of the Lubavitch movement, who died in 1994. He assigned rabbinical emissaries to set up Jewish communities worldwide and directed them to build mikvahs and promote their use. Now there are mikvahs in places like Anchorage, Bangkok and Bogota, Colombia, Rabbi Shmotkin said.
In Easthampton, the new mikvah under construction already has eager would-be users, said Rabbi Leibel Baumgarten, 44, director of Chabad-Lubavitch of the East End.
"There is such a big demand now for a mikvah from people coming out for the summer," he said. "You wouldn't believe how many nonobservant Jewish are calling me asking for a mikvah out here. They realize that mikvah is the foundation of the Jewish people."