A few hundred billion dollars of bad mortgages became trillions in lost wealth — not because the initial loss was large, but because no one could tell where it was sitting.
The proximate cause was straightforward: mortgage lending had extended deep into borrowers who couldn’t reliably repay, and those loans were bundled into securities sold as safe. What turned that into a systemic crisis rather than a sector correction was leverage and opacity — banks held these securities on thin capital cushions, and no one, including the banks themselves, could reliably price what they were actually worth once defaults started.
Markus Brunnermeier’s 2009 account in the Journal of Economic Perspectives identifies the amplification mechanisms directly: as asset prices fell, funding became harder to get, forcing further asset sales into an already-falling market — a feedback loop that turned several hundred billion dollars in mortgage losses into roughly $8 trillion in US stock market value lost between October 2007 and October 2008.
This crisis, alongside the dot-com bubble, serves as historical precedent for reading any new investment cycle skeptically — the specific mechanism differs each time, but the underlying pattern of institutional capital moving faster than real fundamentals recurs (see Institutional Flows).
Source: Brunnermeier, M.K. (2009), “Deciphering the Liquidity and Credit Crunch 2007-2008,” Journal of Economic Perspectives, 23(1), 77–100.