How would Viggo know that Astrid and Throk had used the diving bell in the same manner if he wasn't there to see it .
Astrid and throk were the ones who used the diving bell to go into the volcano not hiccup , fishlegs and mala they rode meatlug into the volcano's entrance , also how did viggo manage to get his hands on it .
There is no eruptodon saliva covering the outside of the diving bell so it should have started melting the second it was placed in the lava .