The ocean once recorded a sound louder than any known animal could make

In 1997, American hydrophones deployed across the Pacific to listen for underwater seismic activity picked up an extraordinary, powerful, very low-frequency sound that got nicknamed "The Bloop."

It was loud enough to register on sensors more than 5,000 kilometers apart — a volume hard to explain with any known marine animal, which fueled years of speculation about some undiscovered, gigantic creature lurking in the deep.

NOAA scientists eventually landed on the most likely explanation: ice cracking, a phenomenon called cryoseism — the sound of massive blocks of Antarctic ice groaning and fracturing, carried by water across enormous distances.

The catch: even though most researchers now treat the ice explanation as the most probable one, the original recording still fascinates people — a reminder of just how little of the ocean depths we've actually listened to compared to land.