Megaliths, alignments, and pre-literate engineering that still resist a clean explanation — the Great Pyramid, Stonehenge, Göbekli Tepe, the Antikythera mechanism, the Indus Valley script.
The four sides of the Great Pyramid of Giza are aligned to the cardinal directions with an average error of about 4 arcminutes — better than 1/15th of a degree. Built around 2580–2560 BCE without iron tools, magnetic compasses, or modern surveying equipment. How its 4th-Dynasty builders achieved that precision is a real open archaeological question; the leading hypothesis (Glen Dash, 2017) is an Indian Circle method on the autumnal equinox, but it isn't the only candidate.
In 1901, sponge divers off the Greek island of Antikythera pulled a corroded lump of bronze from a Roman-era shipwreck. Over the next century it turned out to be a hand-cranked analog computer from roughly 100 BCE — predicting planetary positions, lunar phases, and eclipses with a gear train more sophisticated than anything else known until medieval Europe, fifteen hundred years later.