The ocean.
So incredibly vast. So much so that we know more about our solar system's masses then our own waters. We have the very canyons of Cydonia (on Mars) mapped -- but hardly know what exists 36,000 below.
Creatures of the deep have been long documented. Everything from Cthulu to mermaids. Nowadays, most deep sea findings are dismissed and mythos are disproved.
Until we began listening to the ocean.
The following are a collection of unidentified sounds found by the U.S. National Oceanic and Atmospheric Administration (NOAA), which uses a series of hydrophones set at various points throughout the ocean. I will give a description and a map of each sounds location.
The following sounds are named: "Julia" (my favorite), Slow Down, Train, Upsweep and Whistle.
As you can see, all of these sounds are located within the same uninhabited area. The only island (that's inhabited) even remotely near these sounds is Easter Island.
And here are the sounds themselves:
Julia
(My personal favorite)
Slowdown
Train
Upsweep
Whistle
All sounds occurred in 1997. Except Julia which happened in 1999. Which is a bit odd.
For the specific coordinates and indepth description, you can read the Wikipedia articles; all located here:
en.wikipedia.org/wiki/List_of_unexplained_sounds
What do you think lies beneath the tides? What do these sounds sound like to you?