passages were unearthed during recent renovation work at the hotel, now favoured by foreign tourists and wealthy Vietnamese.
"I felt a little bit like Indiana Jones discovering the Temple of Doom or something," said Kai Speth, general manager of the Metropole Hotel, describing when he first entered the seven-room bunker, which was knee-deep in water.
There were always rumours that the bunker -- no more than 20 square meters (215 square feet) in size -- was under the swimming pool bar, he said.
"So I told the team when we were rebuilding the foundations of the bar: 'let's dig a little deeper'."
The bunker was built in 1968 when the hotel, then known as the Thong Nhat, was a drab, government-run establishment used by the communist authorities to house visiting delegations, including a string of prominent American anti-war activists.
Actress Jane Fonda and folk singer Joan Baez both used the shelter, with Baez recording a song in it during the Christmas bombings in December 1972, when the US dropped some 20,000 tones of ordnance in 11 days.
More than 1,600 civilians died in the attack, and Baez's 21-minute recording "Where Are You Now My Son", made in the concrete passages, captures some of the sounds of wartime Hanoi.
"You can hear the bombs falling. You can hear the anti-air(craft) machine guns going off that were mounted on the Opera House" near the hotel, Speth said.
Fonda arrived after the Christmas Bombings, her then interpreter Tran Minh Quoc told AFP, but was caught in several raids during her controversial tour of the country which earned her the nickname "Hanoi Jane" back in the States.
"We could hear the bombing from afar and we together went down to the bunker... The American air force never hit the hotel. (Fonda) was very calm... She didn't show any fear," he said.
War relics abound for those who look
The bunker at the Metropole, which will be preserved in its original state and is open for tours for guests, is one of thousands of similar bomb shelters dug across Hanoi during the decades-long conflict.
Most have since been filled in, but one other famous site remains -- behind the walls of Thang Long Citadel lies a bunker where former key leaders General Vo Nguyen Giap and president Ho Chi Minh once sheltered from bomb attacks.
While most of Vietnam's war tourism industry is concentrated in the south -- visitors to Ho Chi Minh City can visit the famous war museum and Cu Chi tunnels -- signs of Hanoi's wartime legacy abound if you know where to look.
A small plaque next to Hanoi's Truc Bach Lake marks the spot where US Senator John McCain was shot down as a navy pilot and dragged ashore to become a prisoner of war -- one of 10 American planes to be downed by anti-aircraft gunners on just one day in 1967.
Tourists can also visit the so-called "Hanoi Hilton", where American POWs like McCain were held. While much of the Hoa Lo Prison was demolished, sections of it, including the gatehouse, remain open as a museum.
Not all of Hanoi's war relics are memorials. On the shore of the city's West Lake one family has transformed a former French armaments store into a popular cafe.
"This cafe has a special style because of historic values. When people come here, they are more curious about history," cafe owner Vu Thi Huong told AFP.
In other parts of the city, the wartime history has been absorbed into the scenery.
A small shrine on Yen Ninh-Hang Bun street marks the spot where French soldiers opened fire on a market killing dozens of civilians in an incident that is believed to have triggered the first Indochina War in 1946.
The name of the shrine means 'deep hatred' in Vietnamese and was a popular word used during the war. But today this shrine is famous for another reason -- delicious noodles.
"People here call it 'Hatred shrine' noodle stall," owner Do Thi Yen told AFP.
"Sometime we need to be grateful to the dead. They brought customers to me so I'm grateful to them," Yen added.
A meal down memory lane
Other business owners are hoping to cash in on wartime nostalgia -- a new themed restaurant in Hanoi, called the "State-run Food Shop number 37", takes customers back to the days of food rationing.
For around 25 dollars, you can get an authentic 1970s-style meal for six, served to you by staff wearing uniforms from the actual state-run shops of the period, in a restaurant packed with war-era memorabilia.
"A good place for the old to remember the difficult era and (for) the youth to understand a historic period -- though the dishes are not all delicious," a September review in the official Vietnam News Agency said.
For Bob Devereaux, an Australian diplomat stationed in Hanoi in 1975, wartime relics are important to help the younger generation -- some 60 percent of Vietnam's population are under 35 -- come to terms with the past.
Devereaux was put up at the Thong Nhat hotel, where the Australian Embassy was located at the time, and he used the underground bunker as storage -- even carving his name into the wet cement, where it remains to this day.
"If you find any wine bottles, they're mine," he said laughing, as he showed AFP round the bunker earlier this year after it was reopened, adding that he was pleased the hotel was preserving the space in its original state.
"I'm a great lover of nostalgia so I find it very interesting. It reminds me of what times were like around about 1975. The war was very bitter and savage," he said.
In Vietnam, "anyone younger than 40 or so has no memory of war," he added. "I think it's a good thing for them to come and see what life was about."