Total financial system meltdown? Or a market “correction”?
If the former, then you need a SHTF plan - food, water, shelter, and some store of wealth. Historically, people hold gold, but I’ve always been a skeptic - you can’t eat it, burn it, or convert it to money if there’s no stability anywhere. It’s only valuable if someone else is willing to give you something else of value for it.
If you’re talking about a correction, then the closest thing to cash that you can “invest” in is a money-market fund. If you think the US government is ultimately stable, then a money-market fund that invests exlusively in T-bills is good.
Now, if you have the courage of your convictions, there are funds and ETF’s - “short” funds - that go up when the markets go down. Some are even leveraged where, for example, a 1% decrease in the stock market will get you a 2% increase in fund value.
But, all that said, market-timing (trying to pick and trade market tops and market bottoms) simply does not work in the long run. If you are concerned about the principal value of your investments, then you probably already have too much invested in risk assets. If you don’t need those funds today, in 5 years, or in 10 years (for future retirement, as an example), then you’re better off simply rebalancing your portfolio quarterly or annually to include a mix of stocks, bonds, and “safe” treasuries. A good rule of thumb is to have the stocks be a percentage equal to 110 minus your age. So, a 40-year old planning retirement at 65 would have 70% in stocks (110-40=70).
So I should diversify today and let the chips fall where they may?