Any diversified indexed mutual fund... Of course there's a slight risk involved, but there's a track record.. and if one is worried about the whole market collapsing and not recovering.. well if that happens, he'd have lots of other things to worry about
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Past performance is not indicative of future results.
The question was about "parking" money, which would indicate that the intention is temporary in nature and would not tolerate having less than "parked" at the time person is ready to move it to the investment of choice (which at that time might or might not be an index mutual fund).
"The whole market collapsing and not recovering" is a meaningless statement without a timeframe. The Nasdaq collapsed in 2000 and took many years until it recovered. Any person putting $$ into the Nasdaq on 3/9/2000 and wanting to cash out any time before 2015 would have lost money.
And since the question was about parking money while it grows to critical mass, losing that amount (which wouldn't be that large, and even if it were large) wouldn't necessarily mean that said person would "have lots of other things to worry about". As a matter of fact it might even be a counter indication, as said person might be doing so well financially, that they didn't bother to pull out the small amount at risk.