The Poptropica Creators posted the official Steamworks Island promotional page, and you can find it here:
The only things noteworthy about the page for now are that members will get early access soon and it will become available for everyone to play later this summer. There’s also a short plot teaser, which reads like this:
The silence of empty streets welcomes you. Metal and machines fill the lonely halls, and a growing mystery lurks behind a shroud of steam. Can you uncover what happened to the inhabitants of this ancient island? Gear up for a steam-powered adventure you will never forget!
The look of the images in the promo show vine-covered buildings from ancient times, probably Mayan or Aztec. And the clues in the teaser promo along with the screenshots that the creators have posted show that part of the mystery involves a giant plant monster and that there will be lots of steam-powered machinery involved. And it appears we might even get the chance to pilot a steam-powered mech.