I constantly here people say " Only invest what you can afford to lose"  Well no matter where your wealth is it could crash.

-Invest in USD- USA could collapse and experience mass inflation making your dollars nearly worthless
-Invest in Bonds- Government could collapse making your bonds worthless
-Invest in real estate- Housing prices could crash, natural disaster could hit and insurance company could go bankrupt
-Invest in metals- prices could crash, asteroid mining could collapse prices

Your money is never 100% safe no matter where it is invested.