Efforts to prevent North Korea's relentless advance toward developing nuclear ICBMs have so far achieved little. There is no end in sight to the stream of tests and new missiles and increasingly powerful warheads. If sanctions and diplomacy fail, what else is there?

Secretary of State Rex Tillerson has said that "All options are on the table." But a conventional strike against North Korea's nuclear and ballistic missile capabilities is near impossible. Vital installations are dug hundreds of feet into the sides of mountains, out of the reach of the biggest bunker busters. Now, though, it seems the U.S. has a "game changer," but we don't much about it yet.

The Defense Threat Reduction Agency (DTRA) is a Pentagon organization tasked with combating weapons of mass destruction, including WMDs stored deep underground in concrete bunkers, tunnels, and mountains. In 2015 the agency commissioned a study on Hardened and Deeply Buried Targets (HDBT) by the JASON advisory panel, then a "Counter-Weapons of Mass Destruction/ Hardened and Deeply Buried Targets Game Changer Report" was passed to the Pentagon's Advanced Capability and Deterrence Panel.

"All options are on the table"

This panel, set up a year before the DTRA study, includes joint chiefs of staff, intelligence agencies, and R&D leadership. The panel oversees the Innovation Initiative, which challenges and improves the Pentagon's way of doing things. The clear implication of the study is that the DTRA has some game-changing new technology for attacking deep bunkers.

So what was in the report?

"I can tell you the study examined a variety of classified threats and military requirements and the range of possible approaches and technologies that could meet those areas," a DTRA representative told Popular Mechanics, but the rest remains classified. This is not a lot to go on. If the U.S. government does have an anti-nuke ace up its sleeve, it's well hidden. But looking at the tech developed by DTRA and its sister organizations might help paint a better picture of nuclear deterrence.

GBU-57 Massive Ordnance Penetrator

The DTRA's current biggest and best munition, a 14- ton bomb that's 20 feet long, is able to drive through more than 60 feet of concrete. While the MOP has been upgraded many times, it's not likely that more than slight improvements can be achieved, so it is hard to see this being a game changer.

Giant shaped charge

Detonation of a giant shaped charge at the Tonopah Test Range.

An explosive shaped charge can punch a hole through solid rock so that a follow-on warhead can destroy a deeply buried target. While impressive, this technology is difficult to scale up to a terrifically large size. The 700-pound "largest known shaped charge" developed by Sandia National Laboratories in 2003 was a major engineering achievement and drilled through 20 feet of rock, creating a tunnel large enough for a bomb or missile to follow through. This was at the very limits of engineering at the time, and it is not clear that shaped charges can get any bigger.

Isomer bomb

Some years ago, DTRA funded research into a fundamentally new type of explosive device, triggered by energy releases from a nuclear isomer of the element hafnium, called simply an isomer bomb. Although not as powerful as a nuclear weapon, it could potentially be thousands of times more powerful than TNT. This would certainly be a game-changer—if it worked.

The problem is that many physicists doubt the feasibility of an isomer bomb. While it might be possible to release energy from isomers, it is not clear that the energy could be fast enough to be explosive. As far as we know, the project was canceled because of questions over the validity of the underlying science.

Hard Target Munition

If you cannot make a bunker buster bigger, make it faster. That's the idea behind the Air Force's next-generation Hard Target Munition. Much smaller and lighter than existing bunker busters at 2,000 pounds, the new missile can be carried internally by an F-35 and is rocket-boosted to high velocity, possibly even supersonic speed. However, basic physics limits its bunker-busting ability. Higher speeds tend to shatter the bomb on impact, and increase drag as it travels through rock. It's estimated depth, at tens of feet, wouldn't be enough to reach North Korea's bunkers some 200 feet underground.

Infiltration drones

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Previously the Air Force have looked at swarms of crawling drones to infiltrate underground facilities which could sneak in through air ducts or other entry points. A 2013 Air Force study included "Pipe Snakes" that could enter through sewage lines and burr-like robots that could attach themselves to people entering a facility. Harvard and other organizations have worked on similar swarming drones for non-military purposes, so the tech is possible, but this type of attack likely wouldn't work more than once.

EMP Blaster

If you can't physically destroy a facility, just make it useless. Knocking out all the electronics and the power system with a powerful electromagnetic pulse will put it out of action. The U.S. Army recently developeda weapon called Phaser that destroys circuits with a beam of microwaves, while the Air Force has a similar device called CHAMP which can be carried on a cruise missile. This would take down underground nuclear production plants but would do nothing against stored warheads.

Supercavitating penetrator

Patent for supercavitating penetrator warhead

A DTRA project from a few years ago, this applied the supercavitation principle used by high-speed rocket torpedoes to bunker busting. The idea is that bombs can glide through earth and rock behind a cushion of gas. Little has been published since then, though what little has leaked – like this2012 Israeli paper – suggests that the concept could work.

Deep Digger

A burrowing 2,000-pound bomb, Deep Digger blasts a tunnel through up to 200 feet of solid rock with a set of nose-mounted cannon. The Army developed the tech in 2007, and the plan was for an array of Deep Diggers to be detonated simultaneously to create an artificial earthquake and collapse the deepest bunkers. Prototypes worked well, but nothing has been heard since 2009.

Robotic Underground Munition (RUM)

A cousin of Deep Digger, this 2010 DTRA concept was a robot which could be parachuted into the general area before making its way overground to the target and starting to tunnel downwards, like a more stealthy version of The Mole fromThunderbirds. RUM would also carry defensive systems in case anyone tried to stop it. With a suitable power source, it might get to any depth, but there is no indication that RUM made it past the proposal stage.

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