The answer this this riddle was the pacific sleeper shark!
Meet Somniosus pacificus These sharks are huge! They can get up to an estimated 23 ft long, but more commonly are seen a bit smaller than that.
They are slow moving and take forever to mature and reproduce. There was once a juvenile female found but she was at least 35 years old! Talk about a lame bloomer!
They can be found a few thousand ft deep in Alaskan and Russia waters. And like their cousins the Greenland shark, they can live a long time. Experts estimate upwards of 100 years.