Even if you've been in wine caves before, nothing can prepare you for Palmaz Vineyards' 100,000 square foot underground winery.

By far the largest in Napa Valley, it stretches the equivalent of 18 stories underground from top to bottom. It has four levels, each with separate entrances; one level is an incredible five-store high dome.

The winery is dug into a hillside east of Napa and just under Mount George. It's on 600 acres owned by Dr. Julio Palmaz and his wife Amalia.

In 1978, the Palmazes came to Napa Valley, and in 1996, they bought the forgotten Cedar Knoll Winery, founded in 1881 by one of Napa Valley's pioneer winemakers, Henry Hagen. During Hagen's time, Cedar Knoll was one of Napa's premier wineries and its wines were renowned.

Cedar Knolls' vineyards survived the phylloxera infestation in the 1890s, but Prohibition proved fatal to the business. The winery fell into disrepair and the vineyards were abandoned for nearly 80 years.

The Palmazes restored the impressive old Hagen mansion and their small stone winery while planning new vineyards and the underground winery.

Construction began in 2000. It turned into quite a project, admits Amalia Palmaz. "It wasn't our intention to make it this large. It just evolved," she said. The winery should open to the public on a very limited scale this year. The cave is being built by Angwin-based Glen Ragsdale Underground Associates.

The scope of the cave made construction challenging, especially the digging of the giant underground fermentation dome. The dome is the world's largest underground reinforced structure.

"The challenge for this project was that it is a very large span in weak rock with shallow cover," said Sarah Holtz, a senior engineer with Jacobs Associates, the San Francisco-based firm overseeing the project.

Palmaz, an interventional radiologist and researcher, developed the heart stent used to open arteries clogged by cholesterol. His innovation and attention to sanitation is apparent throughout his winery as well.

The cave dome is 54 feet high by 72 feet wide, cut out of soft lahar flow rock. It consists of fresh andesite boulders in a matrix of highly weathered rhyolite. The dome is supported by 16 feet long, threaded steel rods 1 inch in diameter and a shotcrete lining reinforced with welded wire fabric.

The dome ceiling was finished with hand trowels for a smooth finish to permit movies to be projected on it and was coated with epoxy so it can be pressure cleaned for sanitation.

The floors are tile, and ubiquitous drains allow cleaning. All water is recycled in an underground water-treatment plant to comply with strict conservation guidelines.

The winery maintains a total gravity flow. Grapes arrive on the top level in small bins, pass from the destemmer through a long sorting table manned by six people, then drop to one of 23 fermentation tanks on a rotating carousel 18 feet above the floor. Below them is the press and blending and storage tanks on the bottom floor of the dome, with tunnels used for first-year barrel storage radiating like spokes to an outer circular tunnel.

A large industrial elevator that holds a squat portable tank is used to move wine and other loads to a lower level where the wine is aged a second year. All barrels are stored only one high on stainless racks; the only wood in the winery is the barrels as a precaution against tri-chloroanisole or TCA, the musty compound that makes wines taste "corked."

The caves have a slight slope for evacuating carbon dioxide, but also contain an automatic back-up system to remove excess CO2, but it hasn't triggered after three vintages.

There's also a separate refrigerated arm of the cave to ferment white wines in barrels at a cooler temperature than used for red wines.

The top floor of the underground complex includes offices with windows drilled through the rock, a tasting room, lab and offices. The facility also includes its own bottling line and passenger and freight elevators. The winery has a permit for 14,000 cases (35,000 gallons) of wine per year.

Of course, a winery without grapes is nothing. Palmaz's vines grow in more than 14 different blocks at three elevations on the slopes of Mount George. Their foundation is base rock laid down during the Pliocene volcanic age. The vineyard geography ranges from steep slopes with shallow nutrient-poor soils, which produce concentrated grapes, to stony colluvial deposits made up of cobbles, gravel and sandy loam. Variations of soil type, sun exposure and elevation produce a a wide range of flavors.

The vineyard is planted primarily with cabernet sauvignon, plus some small blocks of merlot, petit verdot and cabernet franc, which are used as blending components for Palmaz wines. Each block is fermented separately, allowing the winemaker to blend wine to satisfy his palate. The winery's production is primarily cabernet, which sells for $100 a bottle. A second label called Cedar Knoll is $45.

Palmaz also produces small amounts of chardonnay, muscat canelli and riesling primarily for its own use. These latter wines are available only at the winery.

Palmaz offers tours and occasional charitable benefits at the Palmaz mansion, rich in local history. For more information, call 226-5587.