February 2.
In the Dentora Region of the Batochiya Republic, before the gate at Mt. Kukurou, a dormant volcano.
The Zoldyck Family's nominal Gatekeepers, Kanboron and Hoso, two middle-aged men—one fat, one thin—watched the two teenagers in front of them, rubbing their hands together in anticipation.
One of the teenagers was Gon, whose arm had just healed, and whose hair stuck up in spikes.
The other was Leorio, who looked rather mature—almost like an office worker—and carried a briefcase.
About twenty days earlier, after obtaining their Hunter Licenses, they'd headed straight for Mt. Kukurou to see Killua, only to be stopped by the gate before them.
The gate loomed like a city wall. Known as the "Gate of the Underworld" and the "Testing Door," it consisted of seven nested gates.
The first gate, the smallest, weighed four tons. Add the second gate and the total was eight tons; add the third, and it became sixteen tons, and so on.
To enter as guests and visit Killua, they had to push open at least the smallest gate—the first one, which weighed about four tons.
At the time, Gon still had his arm wrapped in bandages. He and Leorio strained at the gate with their combined three hands, but soon found themselves stumped by the Gate of the Underworld.
Afterward, they learned more details from Kanboron, the Gatekeeper.
If they tried to sneak in by some other means—climbing over the wall or using a key to open the side entrance—they would be attacked and devoured by Mike, the enormous watchdog.
They'd already seen Mike with their own eyes and knew what a terrifying creature he was.
Left with no other choice, Gon and Leorio decided to follow the Zoldyck Family's rules and enter through the Gate of the Underworld fair and square.