The Lake Ikeda Enigma

Lake Ikeda, Japan, is a caldera lake.  Lake Ikeda's average depth is 135 meters (about 443 feet), while it's maximum depth is 233 meters (about 764 feet).

Lake Ikeda is home to a mysterious creature, Issie.

Here are some sightings of the Lake Ikeda Enigma:

1900, supposedly, marked the first reported Issie sighting.

In 1961, Issie was supposedly seen again. 

On September 3, 1978, 20 attendants of a memorial service testified that they saw Issie.  Yutaka Kawaji was one of the witnesses.  Yutaka Kawaji's children, Hiroto, Mutsumi and Tomoko, noticed two humps from a creature that was gliding through the water.  These were big humps measuring five metres (16.26 feet long) and standing some 60 centimetres (two feet) out of the water.  The children alerted the adults in the party to the presence of the creature and before anyone had time to breathe, Yukata Kawaji leapt aboard a motorboat and pursued the creature as it crossed the lake at a fair clip.

Kawaji was unable to catch up with the creature but during a sighting that lasted three to four minutes, he was able to see the humps twice for about 20 seconds.  Conditions on the lake were mirror smooth with no wind or waves to speak of.  The witnesses agreed that the only activity on the lake came from the immediate area where the cryptid had made it appearance.  Source: http://www.cryptomundo.com/cryptotourism/issie/

Yutaka Kawaji saw Issie on three occasions; twice in one year.
On December 16, 1978, Issie was photographed by Toshiaki Matsuhara.  It was 1.30 p.m. when Matsuhara saw a whirlpool suddenly appear in the middle of the lake.  For five minutes he watched it through a 50X telescope as it moved northward before it disappeared.  While scanning the lake in the aftermath of his initial sighting, Matsuhara’s attention was drawn to a moving object.  He quickly took a series of photographs, and in one of them, what appears to be two humps with spinal ridges, can be seen.  When the photos were published, Yutaka Kawaji contacted the tourism department to say that Matsuhara had photographed the creature he had seen three months earlier and that it was also in the exact same location off a local landmark known as the Couple’s Rock.  Source: http://cryptozoo-oscity.blogspot.com/2009/09/issie-japanese-lake-monster.html

Supposedly, Matsuhara photographed the creature on four more occasions, but few have seen these photos and they have not been submitted for serious scrutiny.                    
Source:  http://www.cryptomundo.com/cryptotourism/issie/

In January 1991, another visitor at the lake took video footage of a creature, approximately 30 ft. in length. 

This video appeared on Nippon TV special World’s Mysterious Phenomena. 
Sources: http://cryptozoo-oscity.blogspot.com/2009/09/issie-japanese-lake-monster.html and http://www.cryptomundo.com/cryptotourism/issie/.

On Episode 18, Season 2, of Destination Truth, Josh Gates and his team went searching for Issie.  Their expedition led to some pretty interesting videos.  Also, they gathered some more sightings of Issie.