Near steep vineyards of riesling grapes, in an underground vault at an air force base in western Germany, sits an American nuclear bomb. More than one of them, actually. Each bomb is about the length of two refrigerators laid down end to end and as heavy as the average adult male musk ox. The bombs are slender and pointy and a little more than a foot wide.

Experts estimate that there are about 100 such bombs stored among five NATO countries, ready to be loaded on jets and dropped by the United States and its allies - old-school style, parachute and all - toward an enemy target. One version of this bomb can carry the explosive equivalent of 11 Hiroshimas.

The bomb's family name is B61. Over the past half-century, in various modifications, B61s have been sent to Europe to deter Russia and reassure the NATO alliance, and they remain there for those reasons. Scenarios for their detonation seem far-fetched - but perhaps not as far-fetched as they seemed a month ago. As Russia's invasion of Ukraine triggers another round of anxiety about World War III, the B61 remains the only U.S. nuclear weapons system based in Europe, a forward-deployed reassurance for NATO at a time when Russian President Vladimir Putin is rattling his own nuclear saber.

The B61 is loaded with meaning. It embodies the paradox, inertia, specter, bargain and cost of nuclear weapons, especially at a moment like this.

Is it ammunition for a hot war or an artifact of a cold one? Both? Neither?

"The political value of these weapons is immense," says Franklin C. Miller, who was President George W. Bush's senior director for defense policy and arms control, referring to the B61s. "NATO governments view them as a major political commitment - the visible, touchable, tangible side of our extended deterrent."

"I remember a chief of staff of the Air Force who asked me if we could get rid of our nuclear weapons in Europe," says Andy Weber, who was assistant defense secretary for nuclear, chemical and biological defense programs under President Barack Obama. "There's no military value to our nuclear weapons in Europe. Zero. They're there for purely political reasons."

The B61 is nevertheless a bomb. It serves a purpose sitting in a vault because it would serve a purpose if dropped from a plane.

"It provides the alliance with a nuclear response - that's its military value," says retired Gen. Philip Breedlove, supreme allied commander of NATO from 2013 to 2016 and now chair of the Frontier Europe Initiative for the Middle East Institute.

How might a B61 be used? During an escalating hypothetical conflict between NATO and Russia, a single nuclear warning shot from Russia into Poland could invite an allied nuclear response: a B61 dropped on a military site in Kaliningrad, for example. This in turn could prompt a Russian escalation, and then - if things continue down that path - all-out nuclear war with the United States, resulting in at least 91.5 million casualties worldwide, according to a 2019 simulation from Princeton University's Program on Science and Global Security.

At that point, we could confront levels of horror that have been confined to theory and fiction for 77 years. If, for example, an 800-kiloton Russian intercontinental ballistic missile detonated 1.8 miles above the White House, there could be half a million fatalities and people might endure third-degree burns from Silver Spring, Md., to Alexandria, Va., according to Nukemap, a modeling website created by nuclear-weapons historian Alex Wellerstein.

U.N. Secretary General António Guterres said on March 14 that "the prospect of nuclear conflict, once unthinkable, is now back within the realm of possibility," referring to Putin's decision to put his nuclear forces on alert.

Appearing Tuesday on CNN, a spokesman for Putin refused to rule out the use of nuclear weapons, particularly if the Kremlin perceives an "existential threat" to Russia.

National security adviser Jake Sullivan noted Tuesday during a White House press conference that it was Putin who, early on, "raised the specter of the potential use of nuclear weapons."

"It is something we do have to be concerned about," Sullivan said. "Based on our current analysis, we have not changed our nuclear posture to date. But we are constantly monitoring for that potential contingency."

What could rouse the B61s from their underground slumber? The classic scenario, Miller says, involves NATO being unable to halt a Russian invasion using its nonnuclear defenses. But the United States has a variety of other, smarter options than the B61 in its nuclear and nonnuclear arsenals. Even in an escalation scenario, the 50-year-old deterrent we keep in the ground might stay there.

"I suppose you could fairly ask me, 'If we were starting fresh, would we need those weapons there?'" Miller says of the B61s. "The answer might be no. But we're not starting fresh, and these weapons have a long history."

The B61 was birthed in the years after the Cuban missile crisis because the Air Force was interested in the possibility of dropping nukes from low-flying aircraft at high speeds, according to the second volume of Chuck Hansen's "Swords of Armageddon." The B61 could be used as a "tactical" or "nonstrategic" nuke on a battlefield, against a forward military target, as opposed to a "strategic" obliterating strike, behind enemy lines, on a seat of government or city.

In the late summer of 1969, scientists and military commanders gathered in Los Alamos, N.M., the birthplace of the atomic bomb, for a three-day symposium on tactical nuclear weapons. It had been almost a quarter-century since World War II ended with a pair of nuclear attacks on Japan and two decades since the founding of the North Atlantic Treaty Organization, or NATO, which by then had grown to 15 countries.

The tactical nuclear program in Europe "remains the single most unifying element in NATO," Gen. David A. Burchinal, then-deputy head of U.S. European Command, said in his remarks. "We must launch a determined program in weapons developments and weapons improvement to meet our present and future requirements," he said. "We cannot rest on the laurels of 20 years of relative calm in NATO Europe."

The new B61 family, in other words, was welcome but insufficient.

By 1975 the United States had in Europe 6,951 tactical nuclear warheads and 145 nuclear storage sites, according to a declassified memo sent to Secretary of State Henry Kissinger. The idea was deterrence: Any Soviet incursion into Europe would risk a limited nuclear strike, which could escalate to an all-out strategic war, which would outweigh any benefits of an incursion, according to James M. Acton, co-director of the nuclear policy program at the Carnegie Endowment for International Peace.

The B61 is left over from those days. It is a bomb that promotes unity by threatening the apocalypse.

"There was a time earlier in my career when I supported the withdrawal of these weapons from Europe," Acton says. "I've changed that because I think there's a number of allies that value them, and I think - especially given recent Russian actions - that they are important enough to a number of countries that I wouldn't want to undermine NATO unity by trying to withdraw them."

The leadership of NATO - of which Ukraine is not a member - has committed to remaining a nuclear alliance as long as nuclear weapons exist.

"At a time when discussions of lethal autonomous weapons, drone swarms and the weaponisation of outer space make modern warfare seem like a sci-fi thriller, nuclear weapons can seem as retro as a Sony Walkman or landline telephone," Jessica Cox, NATO's director of nuclear policy, wrote in 2020. "And yet, nuclear-armed nations such as Russia and China are once again investing heavily to create more sophisticated and diverse nuclear arsenals, North Korea is continuing its nuclear expansion apace, and Iran is once again making headlines for its nuclear developments."

And so the B61 persists, albeit at a fraction of the size of Russia's larger tactical nuclear force, which is undergoing updates and "possesses significant advantages" over the arsenals of the United States and its allies, according to the Trump administration's Nuclear Posture Review in 2018. The estimated 100 U.S. bombs beneath European landscapes are waiting not for detonation so much as refurbishment. (If bombs stick around, bombs get old.) In May, the United States is scheduled to begin full-scale production of a modernized version of the B61 that will have an adjustable yield - meaning that the military can dial up or down the force with which each bomb explodes - and a guided tail kit to improve accuracy (no parachute necessary).

This modernization, which started more than a decade ago, is predicted to cost between $9.1 billion and $10.1 billion - making it probably the most expensive nuclear-bomb program in U.S. history, according to Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists.

"Ever since the end of the Cold War, there have been fewer and fewer" U.S. nuclear weapons in Europe, Kristensen says, and many U.S. officials "say we don't need that stuff there anymore." But for some, he says, the invasion of Ukraine "reaffirms the need for these weapons in Europe," and the modernization of the B61 "commits to the next era of forward deployment of nuclear weapons."

In this way, the life of the B61 evokes the U.S. nuclear arsenal in general: Aged, yet born anew. Mostly hidden but always at the ready. Sacrosanct to some, outmoded to others.

Jon Wolfsthal, senior director for arms control and nonproliferation on Obama's National Security Council, says U.S. officials might wish that the billions spent on B61 modernization were instead invested in nonnuclear capabilities, American troop presence and support for Ukraine.

"When I was in government, we argued the B61 and nuclear-sharing in NATO is essential for alliance unity, right? It turns out it's not," says Wolfsthal, a senior adviser to the anti-nuclear nonprofit Global Zero. "What's essential for NATO unity is the threat of Russia."

The mood in Germany - where for years the parliament has held debates on phasing out custody of those B61s by the vineyards - "has changed significantly since the Russian invasion of Ukraine," says Xanthe Hall, a nuclear-disarmament expert for the German chapter of International Physicians for the Prevention of Nuclear War. "People are actually scared and have woken up to the fact that nuclear weapons threaten them personally."

"The reaction to this is very strong but in both directions. Many people are calling for abolition of nuclear weapons," she says, while "others are saying that nuclear deterrence is our only protection." On March 14 Germany announced that it would replace its aging bomber jets with American F-35s that can also carry B61s, signaling a recommitment to the allies' nuclear-sharing agreement.

Critics of the modernized B61 consider it not a relic of the Cold War or a sign of NATO unity but essentially a new and destabilizing type of bomb, with its "dial-a-yield" capability and increased precision potentially lowering the threshold for use in a conflict, Hall says.

Jill Hruby, the administrator of the U.S. National Nuclear Security Administration, said in a December statement that the B61's refurbishment "improves accuracy and reduces yield with no change in military characteristics, while also improving safety, security and reliability."

While the bombs are being altered, so is the rhetoric around them. In recent years both Russia and the United States have increasingly sent signals about using nuclear weapons in conflict rather than strictly as deterrents to conflict, according to Christine Parthemore, chief executive of the Council on Strategic Risks.

"Around 2015, 2016 I started hearing people in the Pentagon talk about it much more openly, in a way that sounded like they were normalizing it," says Parthemore, who attributes this partly to the belligerent rhetoric of Putin and North Korean leader Kim Jong Un. "If we think nuclear weapons are primarily or entirely for deterring nuclear weapons use, they should be sitting somewhere and maintained as an afterthought, as a political weapon, and seen as not having useful warfighting capability."

But the warfighting capability is what imbues a bomb with its deterrent value, according to experts, and a modernized B61 - with its higher accuracy paired with the low yield - might be a more conceivable option in a military conflict.

"The greatest danger of nuclear war are these so-called smaller, tactical weapons on ambiguous delivery vehicles," Weber says. "Somehow using vanilla terms like 'low yield' makes it seem like they're acceptable."

As Jim Mattis told the House Armed Services Committee in 2018 when he was defense secretary: "I don't think there is any such thing as a 'tactical nuclear weapon.' Any nuclear weapon used any time is a strategic game-changer."

Perhaps as soon as next year, in a world reshaped by however this Russian invasion plays out, NATO soil will be reseeded with modernized nuclear bombs from the United States. They will remain underground and out of sight - but never far from the surface.