"Jughead" was the nickname of a twenty ton hydrogen bomb brought to the Island in 1954 by the United States Army. In 1977, the bomb's primary (fission) device was detonated at the Swan site concurrent with The Incident. ("Jughead")
Under Hostile control
Faraday investigates Jughead. ("Jughead")
In 1954, the US Army set up a tent camp at the Mesa, hanging Jughead from a wooden firing tower in preparation for undisclosed tests. Before these tests could be conducted, the Others, including Richard Alpert and Charles Widmore, attacked the camp, killing all the Army personnel and taking control of Jughead. When a time-traveling Daniel Faraday arrived, he noticed that one of the Others had radiation burns and concluded that the casing of the bomb had cracked. He convinced Richard to allow him to deactivate the bomb, and he was allowed to inspect Jughead under the supervision of his unknowing mother, Eloise Hawking. Upon doing so, he confirmed his suspicion that the casing was cracked and frantically instructed her to patch the hole with lead and then bury the bomb in concrete.
Sawyer asks Richard if he buried the bomb. ("LaFleur")
In 1974, Sawyer asked Richard (who had come to the barracks to confront the DHARMA Initiative after two of his men had gone missing) if his people had buried the bomb. Richard was surprised that Sawyer knew about the bomb, but he did not answer the question. ("LaFleur")
When Daniel returned to the Island in 1977, he said that he believed the Incident could be prevented if Jughead was detonated, halting construction of the Swan. Upon arriving at the camp and demanding to know where the bomb was being kept, Daniel was shot dead by his own mother. ("The Variable") Upon realizing what she had done, Eloise Hawking enlisted the aid of Jack, Richard, and later Sayid to find the hydrogen bomb in order to follow through with Daniel's plan.
Jughead in the tunnels ("Follow the Leader")
She informed them that the bomb had been buried in 1957, on Daniel's advice and was underground beneath the Barracks. Sayid was concerned that she was only interested in the plan as a way to destroy the DHARMA Initiative, however, Jack believed that it was beneficial to them, therefore her interests were irrelevant.
The group came upon a pond near the Others camp where an underwater passage led to a series of "tunnels" as described by Richard (which had architecture similar to the Temple). The hydrogen bomb was found preserved within these halls. ("Follow the Leader")
Sayid removes Jughead's core. ("The Incident, Part 1")
Notes from Daniel's journal helped Sayid to discover that the bomb's core would be all that was needed to create a nuclear explosion. Using the journal as a guide, Sayid removed Jughead's core and placed it in his satchel. Jack and Sayid then escaped the tunnels through the Barracks with Jughead, though Sayid was shot by Roger Linus, despite Sayid's warning that he was carrying a nuclear device. Jack and Sayid were then rescued by Miles, Jin and Hurley in a DHARMA van, who transported them to the Swan construction site, despite resistance from Sawyer, Juliet, and Kate, who eventually agreed to let Jack detonate the bomb. ("The Incident, Part 1")
Jack drops the bomb's core. ("The Incident, Part 2")
Jack volunteered to drop the bomb into the pit made by the Swan's drill, because the bomb was impact-triggered and would go off upon hitting the bottom. After a brief shootout ending in a standoff, Jack dropped the bomb into the pit after the drill hit a pocket of energy, and, with the rest of his friends, awaited their fate. However, nothing happened because the bomb failed to detonate.
Juliet detonates Jughead. ("The Incident, Part 2")
The bottom of the pit suddenly became incredibly magnetized, dragging all surrounding metal objects into itself. Juliet, who was ensnared by some chains, was dragged into the hole, despite Sawyer and Kate's best efforts. Other DHARMA workers were killed by the flying metal objects. At the bottom of the pit, Juliet awakened, somehow still alive, and found the core of Jughead intact beside her. Picking up a rock, she smashed the thermonuclear core eight times before the bomb finally exploded in a flash of white. ("The Incident, Part 2")
Jughead was referenced by Daniel during a conversation with Desmond. Daniel explained to Desmond that he had begun to draw quantum mechanics equations that only an experienced physicist would understand. He explained the purpose of the equations by comparing them to the act of preventing something catastrophic by releasing a large amount of energy, such as a nuclear bomb, to prevent the event from happening. Daniel confesses to Desmond his belief that he has already detonated a nuclear bomb, and ponders if the bomb has caused an alternate timeline. ("Happily Ever After")
- The term "jughead" refers to a foolish or stupid person; Jughead Jones is one of the main characters in Archie comics.
- "Jughead" was the code name for an actual liquid-fuel nuclear device prepared for use as part of Operation Castle, involving a series of nuclear tests which took place at Bikini Atoll in 1954. The actual "Jughead" bomb was part of Operation Castle Yankee. It was scheduled to be the primary bomb at the Castle Yankee test site, however after the success of the solid-fuel bomb at the Castle Bravo test, Jughead was decommissioned and replaced with the more advanced "Runt II" solid-fuel bomb. 
- A picture of the actual Jughead:
- Jughead appears to be a Mark 16 nuclear bomb. The Mark 16 is more properly designated TX-16/EC-16 as it only existed in Experimental/Emergency Capability versions. The TX-16 bomb was 61.4 inches in diameter, 296.7 inches long, and weighed 39,000 to 42,000 pounds. Design yield was 6-8 megatons. Five units were manufactured in January 1954, and deployed in an interim "emergency capability" role with the designation EC-16. By April 1954 they were all retired.
- In the early models of the hydrogen bomb circa 1950, the initial U-235 atomic bomb used to trigger the device is known as the "Swan". This is the small, heavy, explosive-bullet-in-tube atomic weapon that Sayid removes from "Jughead" and manipulates to trigger on impact causing the incident. (The secondary larger U-238 bomb is called the "flute.")
- Did its core detonate? If so, why didn't its radioactivity affect islanders? If not, what happened to it?