An unknown number of nuclear warheads. Stockpiles of plutonium and uranium. Intercontinental ballistic missiles. Weapons factories - and the scientists who work at them.
The list of what it would take for the "complete denuclearisation" of North Korea is long.
North Korea says it's willing to deal away its entire nuclear arsenal if the United States provides it with a reliable security assurance and other benefits.
But there is lingering skepticism ahead of Tuesday's summit between US President Donald Trump and North Korean leader Kim Jong Un that Kim would fully give up the nuclear weapons he has pushed so hard to build.
It wouldn't be hard to hide at least some of the warheads and radioactive materials in the country's vast complex of underground facilities.
Here's a look at the many pieces of a secrecy-clouded bomb program that has rattled the region for decades:
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THE WARHEADS
* Mystery surrounds the size of North Korea's nuclear arsenal - with estimates ranging from 10 to as many as 60 to 70 bombs.
* Their level of sophistication is unclear. North Korea has carried out six underground explosions since 2006 - two of which is it says were hydrogen bomb tests - but it's another matter to make warheads small enough to be sent on a long-range missile to US mainland.
* Last November Kim said his country had mastered that technology. Many foreign experts and governments - including then CIA-Director Mike Pompeo - believe North Korea is at least getting there.
* Analysts believe North Korea is now able to mount nuclear weapons on shorter-range missiles that could reach South Korea and Japan.
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THE INGREDIENTS
* Nuclear bombs can be made from plutonium or highly enriched uranium. North Korea has both.
* A 2016 South Korean government report said North Korea is believed to have produced 50kg of weaponised plutonium, enough for six to 10 bombs.
* In 2007 North Korea shut down the plutonium-producing factory at its main nuclear complex in Nyongbyon as part a disarmament-for-aid deal, but the accord later fell apart, and satellite imagery indicates the North has resumed extracting plutonium in recent years.
* Stanford University nuclear physicist Siegfried Hecker, who visited North Korea's centrifuge facility at Nyongbyon in 2010, recently wrote that North Korea is estimated to have a highly enriched uranium inventory of 250 to 500kg, sufficient for 25 to 30 nuclear devices.
* It takes six to eight kilograms of plutonium to make a bomb the size of a softball, or about 20kg of highly-enriched uranium for a bomb the size of a one litre water bottle, experts say.
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THE MISSILES
* The US would want North Korea to include any intercontinental ballistic missiles in its disarmament steps, as they are the delivery vehicles for nuclear weapons targeting the US mainland.
* Last year, North Korea test-launched three ICBMs that it says are all nuclear-capable. Experts say, though, that North Korea has yet to demonstrate the technology needed to protect its bombs from the severe heat and pressure that a long-range missile is subjected to on returning to the Earth's atmosphere.
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THE FACILITIES
* Most of the key nuclear facilities are at the Nyongbyon complex, about 100km north of Pyongyang. They include a 5-megawatt, Soviet-designed reactor; a radiochemical laboratory where weapons-grade plutonium can be extracted from spent fuel rods and centrifuges to enrich uranium. The closure of uranium mines in North Korea would also have to form part of denuclearising measures, military commentator Lee Illwoo said.
* North Korea demolished its sole nuclear test site in May in front of foreign journalists but experts say this isn't irreversible.
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THE SCIENTISTS
* It's believed 10,000 people in North Korea work in nuclear-related sectors - with 300 of them directed involved in developing nuclear weapons - a 2017 report by Seoul's state-run Nuclear Safety and Security Commission said.
