Triatomic gas in a thinning layer ... and, symbolically, what appears in this puzzle's four longest answers

Question:

Answer:

OZONE

Length:

Similar Crossword Questions:

Number of times the letter 'U' appears in each of this puzzle's eight longest answers
A type of one appears in each part of this puzzle's longest answers
Apt subject for today that's hidden in this puzzle's four longest answers
Peak that's hidden in the four longest puzzle answers
Tampa Bay team playing in this puzzle's longest answers?
Exhibit found in this puzzle's four longest answers
Critter that can follow the ends of this puzzle's five longest answers
Word divided in this puzzle's nine longest answers
They are found in this puzzle's three longest answers
Dream found in this puzzle's seven longest answers
"Dog the Bounty Hunter" network, and a hint to the theme in this puzzle's four longest answers
Number of times this puzzle's theme word appears diagonally in the grid (to complete the puzzle, shade those words plus the Down answers that begin where they meet)
Cash advance taken by this puzzle's longest answers
They're found at the ends of this puzzle's three other longest answers
NFL tiebreakers hidden in this puzzle's nine longest answers
Secret retreat hidden in this puzzle's nine longest answers
Ocean "crossing" this puzzle's four longest answers (abbr.)
Something associated with the first words of this puzzle's seven longest answers
Commercial interruptions literally found in this puzzle's three other longest answers
What you'll find at the end of each of this puzzle's longest answers
Some graffiti signatures (which were used to form this puzzle's four longest answers)
Stocking stuffer found in seven of this puzzle's longest answers
What you might get from the ends of this puzzle's six longest answers
One is hidden in each of this puzzle's four longest answers
Each of this puzzle's four longest answers begins with one