The message bottles are a number of messages that were found some time after the Rokkenjima massacre.
The first bottle, a wine bottle containing several notebook pages, was found by a fisherman on Shikinejima. The pages contained the signature of Maria Ushiromiya, however the handwriting doesn't match the handwriting found on Maria's former belongings. Due to the inconsistency, this bottle was initially thought of as a hoax. However, shortly thereafter, another bottle was recovered from the ocean surrounding Rokkenjima by police. The second bottle contained similar handwriting to the first, lending credibility to both.
Both bottles contained extensive epistolary accounts of the day before and the day of the Rokkenjima massacre. These accounts described the Ushiromiya family being caught up in the witch's epitaph and subsequently dying in inexplicable ways. They conclude with Beatrice reviving and taking everything into the Golden Land. While the two bottles essentially tell the same story, the details of both are completely different. Notably, both bottles list Eva Ushiromiya as perishing when, in truth, Eva is the sole survivor of the massacre.
Some time later, a series of fake message bottles, referred to as forgeries began appearing. These forgeries are fanciful stories written by conspiracy theorists who are fans of the Rokkenjima massacre; the forgeries are their interpretations of what might have happened on the island.