Treasure Isle: Superhero's Sub bonus quest unlocks the deep sea
You can tell from the World Map that the islands of Atlantis (along with the North Pole and the Egyptian Isles) are still off-limits to Treasure Isle players. But its promise is alive, and two indicators of that are the recent Nexus Relic (probably inspired by the Stargate Atlantis sci-fi series) and now, there's the Superhero's Sub, because a submarine's the best way to reach a sunken island city.
To build the sub, you'll need five materials: Gold, Glass, Bolts, Diodes, and Metal. Afterwards, you'll need to recruit your crew, but they don't let you know what that entails until you complete the materials task. You can get the materials by posting a message to your Wall or buying each of them for 1 Island Cash. In total, constructing it should cost you 5 Island Cash, which is pretty cheap, all things considered. But the game indicates that this is for construction of the "Submarine Base", which means you might get another pop-up window asking for more supplies to build the actual sub itself. If that's true, this will suck. Either way, you've got roughly 12 days to find out, because after that, the quest will disappear.