Every six to eight weeks, Gary Weller helps hold a rummage sale that’s unlike any other in Hawaii — and possibly in the United States.

The venue is what’s so unique. The thrift sale benefiting the Hawaii Animal Sanctuary is held in the tall, arched-ceiling underground tunnels of Battery 405 that were part of a World War II coastal defense gun emplacement.

Weller, who acquired the tunnels in 2000, provides tours in conjunction with the fundraisers, which are overseen by the animal sanctuary’s Gina Lay.

During the war, 22 men lived in the Kailua tunnels — at their deepest 350 below the hillside above — and manned two 8-inch naval guns on watch for a seaborne invasion following the Japanese aerial and midget submarine attack of Dec. 7, 1941.

On Saturday, there was definitely a lighter mood.

A circular concrete pad outside the tunnels where one of the big guns once rotated now is armed with a big plumeria tree.

Palm tree lighting fixtures and potted plants flanked the interior entrance to the merchandise area where crystal, china, used clothing, a bowling ball, jewelry, shoes, pottery and other odds and ends were for sale. Rustic colored-glass chandeliers hung from the ceiling of one tunnel, while the intersection of a couple others were furnished with a rug, couch and a fake fireplace.

A yellowed world map that Weller said hung in Gen. Douglas MacArthur’s headquarters in Japan is on display in the foyer. Nearby is a vintage framed portrait of President Franklin D. Roosevelt.

It is here where 23-inch-thick concrete walls meet rummage sale, with a data storage server farm on the side. It’s about as eclectic as it comes for a Kailua neighborhood setting.

Stephany Barajas was making her first visit with husband Richard.

“I never even heard of this until recently on a Facebook (post),” she said. The Honolulu resident struggled to put a defining finger on the experience, but settled on “homey vibe.”

“It’s quirky. It’s fun. I like it. It is different — I can’t explain it,” she said. “It’s not what I expected coming in here, but I like it.”

Richard Barajas said, “It’s very cool how they are preserving it.” He added that early in the war, “just not knowing if the attack was going to happen next and just being on standby and alert — I can just imagine the anticipation, that nervousness.”

The battery was intended to protect what was then Naval Air Station Kaneohe Bay, now Marine Corps Base Hawaii.

Weller, president of Mana Ikaika Inc., is expanding his data center business in the tunnels with a server farm behind an old steel door in a former ammunition storage room in another side to the structure’s re-use.

Two 150-foot tunnels are bisected by shorter connectors. Other tunnels jut off the sides. Altogether, Weller, 68, has 12,000 square feet of underground space.

The backstory of Battery 405 is about as interesting as its current use.

From about 1950 to 2000, businessman Ron Deisseroth grew Japanese mushrooms in the bunker, said Weller, who has a background in computer technology. He said he once visited the operation, which maintained a 98% humidity level and was dark and full of fog.

“In the back there were these orbs floating through the air like spirits, and I said, ‘Oh my God, where am I going,’” he recalled during Saturday’s tour with about 65 people.

The orbs were miner’s headlamps worn by workers who toiled in bathing suits in the foggy environment, he said.

Dozens of Civil Defense wooden crates were stored in a back room and in a locked tunnel, and when Deisseroth moved out, he called several government agencies to alert them. Officials in the early years would regularly check on the emergency supplies but hadn’t visited in years, and no government agency would claim them, Weller said.

Deisseroth had a locksmith open the lock on the steel door, and he eventually sold the lot to Sandii Kama­unu, then the owner of Military HQ on Sand Island Access Road, for $7,500.

Among the items were 55-gallon drums of water and Geiger counters.

There were 200 cots and 200 wool blankets, splints, blood transfusion kits, porcelain bed pans and urinals, and vials of dried-up potassium penicillin crystalline for injections, according to a previous account by Kama­unu. Some of the items bore 1940s dates.

A plywood, fold-out cabinet was full of vintage medications that were all like new in the box, directions included.

Coast defense batteries also were constructed at locations including Punchbowl, Kualoa Ranch, at the Kaneohe naval air station and at Kahe Point.

Weller said he first tried to buy the 19,000-square-foot Battery Arizona at Kahe Point.

“It was in pristine condition when I went there,” he recalled. “A chalkboard still had messages from World War II. The desks still had papers and pens. It was like the men walked out the door and they sealed the bunker and that was it.”

Weller continues to host the rummage sale fundraisers as he pursues his server farm plan. The next one is scheduled for July 6.

“I think Hawaii is so underprepared for a major disaster such as a tsunami, nuclear missile attack, a strong hurricane,” he said. “Everything today is digital. (People) are not backing it up.”