In August 2007, asset-backed securities (ABS), particularly those backed by subprime mortgages, suddenly became illiquid and fell sharply in value as an unprecedented housing boom turned into a housing bust. Losses on the many ABS held by financial firms depleted their capital. Uncertainty about future losses on illiquid and complex assets led to firms having reduced access to private liquidity, sometimes catastrophically. In September 2008, the financial crisis reached panic proportions, with some large financial firms failing or having the government step in to prevent their failure.