Deep beneath a forest in western Lithuania, visitors can step back in time to the Cold War arms race, when the Baltic state was in the communist bloc and Soviet missiles were trained on the West.

Lying in the Samogitia national park are the remains of the Soviets' first-ever underground missile base, opened at Plokstine on December 31, 1962 as tensions raged between the Warsaw Pact and NATO.

Visitors have to use their imagination to paint a picture of the site in its heyday.

Moscow removed the equipment when it shut down Plokstine in 1978 to pave the way for the following year's SALT II arms-reduction treaty with Washington.

Red Army troops left Lithuania after the country won independence from the crumbling Soviet Union in 1991, and metal-thieves later stripped the site.

But the scale of what is left remains breathtaking, notably the silo -- one of four -- which is open to tourists.

There, visitors squeeze through a trapdoor into the 27-metre (89-foot) deep, five-metre (16-foot) wide underground tube which was dug out by hand by some of the 10,000 Red Army soldiers who built the base.

"The depth is really impressive. Somehow it feels like everything's vibrating below your feet. I wanted to get out as soon as I could," said Giedre, a young mother visiting with her three children.

The base was home to four R12-class ballistic missiles -- known to the West as the SS-4. Each had an explosive power of one to two megatonnes, dozens of times more powerful than the atom bomb that destroyed Hiroshima in 1945, the site's guide explained.

The missiles' range was around 2,000 kilometres (1,200 miles), enabling the Soviets to strike from Plokstine at almost any nation in Europe. The targets were adapted every three or four years, in line with political tensions.

"The only really critical moment was during the events in Prague in 1968," former Red Army officer Ricardas Valeckas told AFP, referring to the East-West tensions over the Warsaw Pact's military clampdown on reform-minded communists in Czechoslovakia.

"The level of alert was raised, and we were on duty, waiting for the signal," said Valeckas, who worked at the base from 1964 to 1978 and was the only Lithuanian with access to its heart.

Plokstine was in a high-security zone, ringed by a 1,700-volt electric fence, and restrictions were regularly imposed on local residents.

"Sometimes they ordered people to close their curtains and switch off the lights, but we knew that missiles were being delivered because the ground would vibrate," said Valeckas' wife Regina, who is from the region.

"In any case, people used to peek through the window. In fact, the locals were more aware of what was going on than the soldiers," she said, laughing.

Keeping the base secret from NATO was also a tall order, because Soviet territory was regularly scanned by Western spy satellites.

"When we held exercises or were doing maintenance work -- like cleaning the nose-cones of the missiles -- we kept an eye on the satellites' orbit times. And sometimes we put things on hold until they had gone past," recalled Valeckas.

He said he was always convinced that the missiles would never be used, even though the Soviets insisted the base was a crucial link in the bloc's defensive chain.

For Valeckas, Moscow's bombastic language about keeping pace with the West was in stark contrast with the daily lot of ordinary Soviet citizens.

"We were completely aware of how we were lagging behind technologically. They wanted us to overtake America, and we didn't even have toilet paper!" he said.