Every six weeks or so, Gary Weller unlocks the door to Battery 405, a World War II coastal defense facility hidden in the hills overlooking Marine Corps Base Hawaii, and whisks visitors more than 70 years back in time.

He offers a 45-minute tour of the historic bunker, located on his 3-acre property, in conjunction with rummage sales benefiting the nonprofit ­Hawaii Animal Sanctuary founded by his close friend, Gina Lay. It’s an interesting way to spend a few hours on a Saturday: Find great deals to support a worthy cause and learn about World War history in a 12,000-square-foot relic that’s open to the public only about eight times per year.

“In the 1940s, the Marine base was Kaneohe Naval Air Station,” Weller said. “On Dec. 7, 1941, Japanese Zeros destroyed the seaplanes there before going to Pearl Harbor. After that horrific day, it was decided the naval air station needed more protection in case the Japanese attacked again.”

Construction of Battery 405 started in early 1943 and was completed a year later at a cost of about $284,000. Its two eight-inch Mark VI M3A2 guns could shoot 20 miles out to sea.

“When the guns were fired, the shock waves were so great, they cracked the water tower on top of the hill,” Weller said. “All the farmers in windward Oahu used water from that tower, so the mayor asked that the guns not be fired unless the air base was actually being attacked. The guns wound up being fired just three times each as tests, because the Japanese did not attack Hawaii again.”

A roof was built over each gun, and from the outside the bunker itself looks like a house with a chimney. The idea was if the Japanese took aerial photos for reconnaissance, they would mistake the structures for a Japanese fishing village and rule them out as targets.

World War II officially ended on Sept. 2, 1945. From 1950 to 2000, Battery 405 doubled as a mushroom farm owned by Ron Deisseroth, an entrepreneur, philanthro­- pist and community leader who, among many other positions, served as president of the Rotary Club of Waikiki, chairman of the board of the Waikiki Improvement Association and a board member of the American Heart Association and Multiple Sclerosis Society in Hawaii.

Weller acquired the bunker from Deisseroth in 2000 to house Mana Ikaika, his data storage business; that sparked his interest in World War II history.

“When Mr. Deisseroth got the bunker, it was in great condition because it had hardly been used,” Weller said. “But growing mushrooms took its toll. Mushrooms need moisture, and water was pumped into the bunker every day. That eventually damaged the wood, paint, pipes and conduits. It took five years and a lot of money to do repairs and renovations, including removing 33 dump truck loads of scrap wood, broken bricks, corroded pipes, rusting equipment and other debris.”

Battery 405 turned 75 this year, and it remains a veritable fortress. Its walls are 23 inches thick, its curved ceilings are 19 inches thick and it sits on a 4-foot-thick cement floor. Weller’s tours go through two 150-foot tunnels and shorter connecting rooms. There’s no air conditioning, but the interior temperature is a comfortable 62 to 76 degrees Fahrenheit year-round.

When the military occupied the bunker, walls were erected to create rooms for storage, bunks, an emergency generator, fan and air-conditioning equipment, and gun, radio and fire controls. Space was also allotted for latrines, showers, a kitchen and a first-aid station. The original arched ceilings were covered with flat ones.

“The reason for that was a man might not know he’s claustrophobic, and when he’s in the bunker and sees the curved ceiling, he thinks about being in an enclosed facility,” Weller said. “He might panic and that’s the last thing you want to happen in a place that stores a lot of explosives. If they ignited, there’s no way you could put out that massive fire with any amount of water. Twenty-two men were stationed there round the clock, and flat ceilings made them feel like they were in an office rather than a bunker.”

Weller is an animated storyteller; even so, a good deal of imagination is needed to visualize what some areas of the bunker looked like in the 1940s because they’re now crammed from floor to ceiling with tools, machines, equipment and supplies. One former projectile room houses Mana Ikaika’s servers; the other is now Weller’s office.

Dozens of World War II-era photos are exhibited at the entrance along with an item of special note. On the wall behind portraits of President Franklin D. Roosevelt, who was in the White House during the war, and Dwight D. Eisenhower, commander of the Allied forces in Europe, is a 36-by-56-inch map of the world that was gifted to Weller by the son of the man who headed Gen. Douglas MacArthur’s mapmaking team in Japan.

As a guest on one of Weller’s tours, the son mentioned that his late dad, a resident of nearby Enchanted Lake, had created maps for MacArthur, who oversaw the Allied forces in the Pacific theater and the post-war U.S. occupation and reconstruction of Japan. The map in the bunker had been displayed in the general’s Tokyo office, then given to the mapmaker, who bequeathed it to his son. It is yellow, in part from age but also from tobacco.

“People smoked a lot back then, and MacArthur loved his corncob pipe,” Weller said. “I called the U.S. National Archives to see if the tobacco stains could be removed and was told there was no way to do that without destroying the map. So I’ve kept it as is for people to see. It’s a valuable artifact that represents an important chapter in American history, and it should be preserved and shared — just like the bunker.”

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COMING UP

The Hawaii Animal Sanctuary’s annual Sip ’n’ Shop fundraiser will be Dec. 7, from 5:30 to 10 p.m. at 221 Iliaina St., Kailua, near Aikahi Shopping Center. The $5 admission includes a beverage and appetizers. Reservations are not necessary.

Available will be holiday-themed gifts and jewelry in addition to the usual rummage sale mix of books, music, clothing, crafts, furniture, housewares and more. Parking is limited; please carpool. Bunker tours will not be offered that evening.

The animal sanctuary’s year-end sale will be Dec. 14, from 8 a.m. to 2 p.m. Gary Weller will be leading tours at 10 and 11:30 a.m. and 1 p.m. Cost is $5 for adults and $3 for children aged 5 through 12.

Next year’s dates for rummage sales and tours will be posted at facebook.com/HawaiiAnimalSanctuary. Private tours for schools, community organizations and military veterans can be arranged.

For more information, call Weller at 253-0675.

Cheryl Chee Tsutsumi is a Honolulu-based freelance writer whose travel features for the Star-Advertiser have won several Society of American Travel Writers awards.