What is The Jamaica Thing in the new Pretty Little Liars book Twisted?

In the new Pretty Little Liars book Twisted, "The Jamaica Thing" refers to an unresolved traumatic incident that involves the disappearance of Ali, Spencer, Aria, Hanna, and Emily on the island. While the details about what exactly happened in Jamaica remain cryptic and unfold throughout the book, this incident is seen as the catalyst for their troubled relationships.