[cynwoody]

It's not useful for mag tape or wire recordings, but archaeologists have available a technique that can date artifacts like campfires, pottery kilns, and burned out adobe houses within the last 10,000 years.

It's called archaeomagnetic dating[0]. It turns out that heated ferromagnetic materials, such as magnetite, capture the magnitude and direction of the earth's magnetic field as they cool down through the Curie temperature[1]. That allows an investigator to ascertain the direction of magnetic north the last time a likely sample was heated above the Curie point. Over time, magnetic north changes with respect to true north.

    [0]https://en.wikipedia.org/wiki/Paleomagnetism
    [1]https://en.wikipedia.org/wiki/Curie_temperature