You take out a 100k mortgage in USD. Over the next year, the USD suffers suffers from extreme deflation (increases in value). It is now worth twice what is was worth last year. As a consequence, wages begin to go down. Now let's say you were earning 50k a year while paying down a 100k mortgage (not assuming a downpayment, this is for simplification). Next year, you are now being payed 25k but still owe that 100k mortgage. And the house is now worth 50k.