Terrorists plotting to simultaneously explode several planes in midair from London to America using 'liquid' bomb materials.
British Authorities knew about plot and was investigating it for sometime before events necessitated arresting plotters.
21 plotters arrested, more sought.
"A senior U.S. counterterrorism official said authorities believe dozens of people — possibly as many as 50 — were involved in the plot, which "had a footprint to al-Qaida back to it." The official spoke on condition of anonymity because of the sensitivity of the situation."
From British Authority: "Attempt would have been mass murder on an unimaginable scale."
New rules in effect for major airlines. No liquids, electronics and severly limited carry ons to be allowed in the future.