You can get into the Magma Pool at Moltara, which is directly above Lutari Island and beside Maraqua. Once you reach Moltara, click on the cave on the right side which will lead you to the other part...
There is nothing inside it as such. It is a Magical Morphing Experiment. If bought the item will change over time into different wearable items. There have been other similar morphing NC items.
Most likely, it is fermenting. The sugars or starches are being consumed by bacteria. It will result in carbon dioxide, and ethanol alcohol. The carbon dioxide is the bubbles yous see. Vinegar...