This 2014 video says about itself:
Narwhal Whale – Fun Fact Series EP43
The tusks measure 7 to 10 feet in length.
Narwhals swim belly up and lie motionless for several minutes and this has earned them the name, “corpse whale.”
Narwhals use their forehead to feel sound waves and hear each other as they bounce through water.
From Science News:
Narwhals are really, really good at echolocation
by Helen Thompson
11:33am, November 11, 2016
Narwhals use highly targeted beams of sound to scan their environment for threats and food. In fact, the so-called unicorns of the sea (for their iconic head tusks) may produce the most refined sonar of any living animal.
A team of researchers set up 16 underwater microphones to eavesdrop on narwhal click vocalizations at 11 ice pack sites in Greenland’s Baffin Bay in 2013. The recordings show that narwhal clicks are extremely intense and directional — meaning they can widen and narrow the beam of sound to find prey over long and short distances. It’s the most directional sonar signal measured in a living species, the researchers report November 9 in PLOS ONE.
The sound beams are also asymmetrically narrow on top. That minimizes clutter from echoes bouncing off the sea surface or ice pack. Finally, narwhals scan vertically as they dive, which could help them find patches of open water where they can surface and breathe amid sea ice cover. All this means that narwhals employ pretty sophisticated sonar.
The audio data could help researchers tell the difference between narwhal vocalizations and those of neighboring beluga whales. It also provides a baseline for assessing the potential impact of noise pollution from increases in shipping traffic made possible by sea ice loss.