1. Loud enough to shake the very earth 
2. Sufficiently significant to affect the whole world
   "earthshaking proposals"
   - world-shaking, world-shattering, earth-shattering

See also: important, loud, significant

• forever and a day
• forevermore
• forewarn
• forewarning
• forewing
• forewoman
• foreword
• forex
• foreyard
• forfeit
• forfeited
• forfeiture
• forfend
• forficate
• Forficula
• Forficula auricularia
• Forficulidae
• forgather
• forge
• forged
• forger
• forgery
• forget
• forget me drug
• forget-me-not