The Diving Bell is a contraption built by Hiccup in Race to the Edge, Season 4. It was originally made for getting underwater to free Shipsbane the Submaripper from the Straits of Baldur in the episode, Dire Straits.
Hiccup got the inspiration to create the diving bell from Tuffnut and Ruffnut goof off with a smaller cauldron on Ruff's head. He built it from Gobber's old smelting cauldron by flipping it upside down and had it reinforced with Gronckle Iron and Death Song amber. He also attached legs to keep the bottom off the floor. He then used it to dive to the sea floor to free the trapped Submaripper by using Changewing acid to melt the chains. It took a few tries to get it air tight, however, as the amber cracked under the water pressure, nearly drowning Hiccup. Once it was safe, he used it as an area to take a breath while swimming back and forth from the Submaripper, and melting its chains. When the dragon was riled up by Viggo's catapulted boulders, it suddenly knocked the diving bell, breaking its legs. A boulder then hit the ship's pulley system that was raising it to the surface, sending it back to the ocean floor with Hiccup trapped inside. Even Toothless was unable to free his friend from the sealed diving bell. Shipsbane however must have sensed the trouble and as thanks for freeing it from its chains, brought the bell to the surface and dropped it onto the deck of the ship, where, with the help of Barf and Belch, Hiccup was released from its casing.
While Hiccup, Fishlegs, and Mala were attempting to get the Eruptodon egg to its nesting area inside the Defenders of the Wing's volcano on Meatlug, Astrid and Throk worried that it was taking them too long to get back. So Tuff came up with the idea to use the diving bell as a way to lower them in safely, so they can rescue their comrades before the lava rose too high. It was then coated with Eruptodon saliva, like the armor that the Defenders of the Wing wear, for protection against the immensely hot lava and with Barf and Belch's help, safely lowered Astrid and Throk into the volcano to rescue them. The chain holding it melted and broke, however, sending the bell into the lava lake, with Astrid and Throk stuck inside. The heat resistant Eruptodon saliva was slowly being worn off, and the duo started getting overheated and delusional. Thankfully Meatlug reemerged with her three riders, rescuing Astrid from the bell. The Great Protector then appeared and grabbed the chain, lifting the diving bell, with Throk still inside, back to safety.