Paper currencies, yup, some have failed.

Do we sink to the bottom of the ocean if the currency collapses?

Devaluation of paper currency.

Learn about crypto currencies and how to trade them.

If only I had done more USA investing when the pound was two to one (I mention this because currency and tax have started to become as critical to my decisions as other factors).if you want exposure to Europe it's probably easier via equities or currencies unless you are keen to learn all the intricacies of the local markets.

I have reserves in both currencies. 6 months in AUD, a little bit more in USD. 

The dollar is still the most powerful currency on earth.most currencies have crashed and stayed down (look at the euro for example)The FED printing money is mostly going back into the markets which doesn't affect inflation too much.A 2k let person stimulus while no one is spending will likely do nothing to the value of the dollar.With oil down so low, the dollar just got even more powerful

When a currency collapses, all that remains is all that remains.

The only currency that will matter will be food, water purification systems, guns, ammo, and antibiotics.  

I got 15% note rate and then another big move with the currency arbitrage.. although for me lending in BC I did not research it far enough it ended up well as stated.. but once I dug into what happens if they don't pay.. that's when I got a little bit of an ut oh not the US and much tougher.. it was a small number like 100k.. so not the end of the world but still.