Deep-time geophysical mysteries — the Bloop and other unidentified hydrophone events, anomalous quakes, the Yellowstone caldera question, magnetic excursions, the planet's quieter weirdness.
In 1997, NOAA's Pacific hydrophone array picked up an ultra-low-frequency sound powerful enough to register across sensors more than 3,000 miles apart. The signal was named the Bloop. For fifteen years it sat in NOAA's unidentified catalogue. In 2012, Christopher Fox publicly attributed it to a non-tectonic icequake — a large iceberg cracking off the Antarctic ice shelf. The rest of NOAA's unidentified catalogue (Julia, Upsweep, Train, Whistle, Slow Down) is still active.