Complex sentence: Throughout the island people have found mysterious artifacts such as ancient coins, discovered flood shafts, strange glyphs on rocks, and repeated excavations have even reported layers of apparently human-made artifacts as deep as, but ended in collapsed excavations and flooding.
Split into simpler sentences: Throughout the island people have found artifacts such as old coins, discovered flood shafts, and strange glyphs on rocks. Repeated excavations have even reported layers of apparently human-made artifacts as deep as, which ended in collapsed excavations and flooding.