Boiling Sea

The Boiling Sea is a dangerous body of water located somewhere presumably in the Endless Ocean. It is a series of undersea volcanos that engulfed the once thriving port town of Hono Mizu. Inside the largest volcano is where the town is currently located at. Inside the town is an ancient library, which Ray and Maya used as a shelter to hide away the Reversal Time Blade under the orders of Wu after he discovered the weapon in the wood near the Monastery of Spinjitzu.

History
Following the defeat of Krux and Acronix after their powers of Time where absorbed by the Time Blades, the Time Blades were used by Wu and Garmadon to open a Temporal Vortex, where they used it to dispose of the blades, but Krux and Acronix soon followed them, where they became lost in time. However, Krux would somehow immediately escape, landing in the woods below the monastery and fleeing, unaware of the Reversal Time Blade, which landed directly after he fled.

Shortly after this, Wu discovers the Time Blade in the woods. Knowing it was too powerful to keep, he ordered Ray and Maya to dispose of it within the Boiling Sea, knowing only the current Elemental Masters of Fire and Water together could reach the blade through a series of trials.

Pause and Effect
After Kai and Nya reunite with their parents, they explain to their children on what Krux and Acronix's master plan was; to create a time-traveling mech known as the Iron Doom, which would allow them and their Vermillion warriors to travel to any point in Time they please. After Nya realizes that the pause they felt earlier was the arrival of the third Time Blade, Kai realizes that if Krux and Acronix found the last one, their mech would be fully operational. However, Ray and Maya explain that Wu found it shorty after Krux and Acronix's defeat, where they were ordered by him to dispose of it within the Boiling Sea.

Maya then realizes that because their children are the current masters of Fire and Water, they could be used by The Hands of Time to retrieve the blade. Just before they could leave, they are confronted by Krux and Acronix, where they explain that Kai and Nya will recover the last blade for them, otherwise the dying Master Wu will perish.

Out of the Fire and Into the Boiling Sea
Kai, Nya, Ray and Maya, along with Krux, Acronix, Machia and a dying Master Wu, use Machia's airship to reach the Boiling Sea. After Ray and Maya explain the history and what awaits them within the Boiling Sea, they use the Dragon Blade to create the Fusion Dragon, where they use it to traverse the Boiling Sea to reach the city of Hono Mizu.

After a series of trials and a fight with several Geoatomic Rock Monsters, Kai and Nya are able to recover the Reversal Time Blade from within the library. As the who place begins to fall apart, they summon the Fusion Dragon to return to Machia's airship. However, Krux and Acronix reveal they had no intentions of letting them use it to save Wu, enabling a battle. In the end, Acronix strikes Ray with the Forward Time Blade and then pauses the group with the Pause Time Blade. As Krux claims the fourth blade, Acronix orders Machia to dispose of the group at the Boiling Sea. As the airship leaves without them, the effects of the Pause Time Blade wears off, causing everyone to plummet down towards the sea. Kai and Nya then summons the Fusion Dragon again and picks up the others before following Machia's airship back to the Vermillion Swamps.