Surrounded by the rising heat of steaming volcanic rocks — in darkness so thick, even their own hands are indistinguishable from their neighbors' — the sweat begins to pour.

Slowly, Abraham, Sarah, Isaac, Rebecca and the other forebears are invited to enter the sukkah-like lodge, an igloo-shaped structure made of willow branches. The top is covered with blankets, one of which bears the Star of David.

As chanting and singing fill the lodge, more rocks are piled into a pit at the center and stoked by droplets of water. The steam increases and the 20 participants from the Oakland-based Aquarian Minyan — all sitting in a circle around the rocks, just days before Yom Kippur — surrender to God.

"The High Holy Days are for peeling off the extra layers we accumulate each year and don't need to be carrying," said Yehudit Goldfarb, a founding member of the 26-year-old Aquarian Minyan and a sweat lodge participant.

"The lodge is another level of that."

Set amid the Days of Awe, the Minyan's annual purification lodge is "a cleansing of both the body and spirit," said Miriam Stampfer, a biologist at U.C. Berkeley.

"You purify yourself on the physical level, by cleaning the toxins out of your system," she said, "and on a spiritual level, by following ritual paths, which bring in the spirits of the ancestors and the divine presence."

It will be held this year on Thursday.

Describing the "anti-superficiality" that the darkness of the sweat lodge creates, Goldfarb's husband, Reuven, added: "You're dropping the mask, dropping your social persona and bonding on a higher, more unifying level. Each person is alone, together. Out of that commonality emerges something very beautiful."

Aquarian Minyan board chair Karen Roekard first developed the sweat lodge idea nine years ago, after meeting a Native American neighbor who leads the purification ritual in her Berkeley backyard. It reminded her of another cleansing ritual from generations past: the shvitz.

"I remembered when I was a kid and my father would go to the shvitz, sweat out whatever angst was going on in his life and come back renewed," said Roekard. "So I got together with my neighbor to see if we could combine the Native American and Jewish cultures into a special High Holidays sweat lodge."

The ritual was tailored to fit Jewish ideas but draws from Native American tradition.

"It is not free form," explained Stampfer, who also participates as a dancer in Sundance, a Native American ritual organized by a Black Elk descendant. "You have to do it just so — it is very easy to do the wrong thing."

For this reason, Roekard's neighbor, "a special teacher of the Native American tradition, who explains in a sweet, sweet voice what to do" leads the sweat, said Roekard.

While a sweat lodge seems a radical departure from conventional Jewish practice, it actually "taps into what's inherent," Yehudit Goldfarb explained.

"Judaism was once grounded in earth rituals and physicality," she said. "This is a good way of drawing into our own tradition."

Stampfer agreed. She drew connections between Jewish and Native American rituals, calling them both "land and seasonal based."

The sweat lodge is actually just one event in a month's worth of celebration for the Aquarian Minyan.

The celebration, or "Tishri Experience," includes Rosh Hashanah and Yom Kippur services, "Open Sukkahs" at members' homes, and a weeklong schedule of "Teshuvah Nights" — small, themed gatherings that follow Rosh Hashanah and lead up to Yom Kippur.

"It's three and a half weeks mixed with meditating and partying," said Roekard, explaining that in ancient times, Rosh Hashanah and Yom Kippur were merely buildups to "the big fall harvest blow-out" of Sukkot. The month then culminates in a major closing celebration at Simchat Torah.

Roekard compared the Teshuvah nights to "gate-openers, preparing the heart for the purification, connection and healing of Yom Kippur."

"Every night of the week, someone talks about some topic of interest, geared a little for the head, a little for the heart and a little for the soul. These smaller gatherings where people can come and meet one another have helped the community on a gazillion levels."

This year, for the first time in the Aquarian Minyan's history, the celebrations, including the sweat lodge, will be open to the general public. "We've always kept it private within the community," said Roekard, who is former program director at Congregation Kol Shofar in Tiburon. "It's time to really open our doors and let some new energy in."

To secure a spot in the sweat lodge, however, reservations are required. Spaces are limited and participants must be both experienced with sweat lodges and in excellent physical and emotional health.

"It's hot," said Goldfarb, "Really hot. You go to a place of not knowing what your endurance is."

"But with each new level of heat comes another level of focus. And with that a sense of God's sovereignty."