The past is full of strange events, and some have left experts guessing. This fascinating series pairs short paragraphs of easy-to-read text with plentiful photos to help readers explore some of these mysterious occurrences, highlighting what happened and what people still don't know. Books also include a table of contents, fun facts, sidebars, comprehension questions, a glossary, an index, and a list of resources for further reading. Apex books have low reading levels (grades 2-3) but are designed for older students, with interest levels of grades 3-7.