You are looking for plastic jar similar to a jar from peanut butter.
Cache is along the trail called Cabin John Trail in a local park.
Cache is close to the trail so it is bike friendly.
Why this name?
Dark Blue World is a name of movie from 2001. The film is about Czech pilots who fought for the British Royal Air Force (RAF) during the Second World War. The film is written by a very good screenwriter (Zdenek Sverak) and directed by his son (Jan Sverak, who also won Foreign Film Academy Award for film Kolya; done by the same father-son duo)
More about the movie: March 15, 1939: Germany invades Czechoslovakia. Czech and Slovak pilots flee to England, joining the RAF. After the war, back home, they are put in labor camps, suspected of anti-Communist ideas. This film cuts between a post-war camp (where Franta is a prisoner) and England during the war (where Franta is like a big brother to Karel, a very young pilot). And there is also a love story....