(VIDEO) Scientists monitoring the oceans have uncovered a handful of sounds that can't be explained — at least not with any certainty.

With names like "The Bloop," "Train" and "Julia," the sounds have been captured by hydrophones, or underwater microphones, monitored by the National Oceanic and Atmospheric Administration (NOAA). Here are the six most mysterious noises ever heard in the sea, and the best (but not proven) explanation of what might have made them.

1. The Bloop

The decidedly nonspooky nickname for this sound does little to dispel
the mystery surrounding it. In 1997, NOAA hydrophones picked up one of
the loudest sounds ever recorded off the southern coast of South
America: the Bloop (which sounds like, well, a bloop), was recorded by
two hydrophones nearly 3,000 miles (4,800 kilometers) apart.

The Bloop mimics marine animal sounds in some ways, but its volume is
too great to be made by any sea creatures known to science. If your
imagination is running away from you, you're not alone: Plenty of
listeners have jokingly linked the Bloop to Cthulhu, a fictional
part-octopus monster created by sci-fi writer H.P. Lovecraft in 1928.


Deep-sea monsters aside, NOAA holds the most likely explanation for The Bloop is that it was the sound of a large iceberg fracturing.
These "icequakes" have been recorded in the Scotia Sea and sound very
similar to the mystery 1997 Bloop. If a cracking iceberg were the
source, according to NOAA, it would have likely been floating between
the Bransfield Strait and the Ross Sea of Antarctica, or perhaps at Cape Adare in East Antarctica.

2. Julia


This weird noise, which sounds almost like someone cooing or whining,
occurred on March 1, 1999. The eastern equatorial Pacific autonomous
array (a network of hydrophones) picked up this strange sound.

Like the Bloop, Julia is most likely the sound of ice. In this case,
NOAA researchers suspect the hydrophones picked up the sound of a large
Antarctic iceberg running into the seafloor.

3. Upsweep

This sound is like the scratch of branches against your bedroom window,
in that it happens again … and again … and again. To the ears, Upsweep
sounds like an ambulance wail or perhaps an unearthly creature's howl.
It's been picked up by hydrophones seasonally since 1991, peaking in the
spring and fall. The source of the sound appears to be an area of undersea volcanic activity, but scientists have yet to pin down exactly what's causing it.

4. Slow Down

Slow Down, a noise recorded on May 19, 1997, gets its name because it
descends in frequency over seven minutes. NOAA scientists have located
the source of the sound off the Antarctic Peninsula, leading them to
suspect that Slow Down is the result of a drifting iceberg hitting the
seafloor and screeching to a ponderous stop. The sound was detected by
sensors nearly 3,100 miles (5,000 km) apart.

5. Train

Train sounds like you might expect a noise named train to sound — like
the rub of train wheels against tracks. Recorded in 1997, Train is a
steady hum that likely originated in Antarctica's Ross Sea. The
suspected culprit? An iceberg dragging its keel along the ocean floor.

6. Whistle

Whistle sounds more like a kettle of boiling water than a jaunty tune,
but that doesn't make the sound any less mysterious. Recorded in July
1997, Whistle was only picked up by a single hydrophone, making it
impossible to pinpoint its source. However, according to NOAA, similar
sounds have been recorded coming from erupting submarine volcanoes. It's possible that Whistle, unlike Julie, Slow Down or other mysterious
sea noises, may have been caused by fire, not ice — though we won't
blame you if you want to imagine it was Cthulhu after all.