Crossword Clue:

that cannot be found

Answer	Crossword Clue
ASTEROID	Something that may be found in a belt
OBOE	Something that may be found in a pit
EMAIL	Letters that can't be found at the post office

AERIE	Nest that may be found on a 1-Across
DEERXING	Road sign that hints at what can be found three times in this puzzle's grid
ARGO	Unmanned vehicle that found the Titanic
SETI	Grp. that hasn't yet found what it's looking for
OPAL	Precious stone that has been found on Mars (fun fact!)
BORON	Element with a low atomic number that is not found naturally on Earth