The bursting of the housing price bubble during 2007 and 2008 was accompanied by high interbank spreads, and a partial breakdown of interbank lending. This paper theoretically models how Knightian uncertainty over banks risk exposures may have contributed to the breakdown. The paper shows: 1) the two-tier structure of the U.S. Fed Funds market makes it robust to uncertainty, but the market may nevertheless collapse - and private incentives to restart it may be insufficient. 2) In some circumstances government bank audits and information releases about exposures that resemble a stress test can restart markets and improve welfare by internalizing an externality associated with economy-wide uncertainty reduction. 3) Collapses due to uncertainty are less likely ex-ante and less costly to fix ex-post when there is better publicly available information on core banks aggregate risk exposures. Based on 2) and 3), ex-ante and ex-post "transparency initiatives" are proposed. Their success depends on the financial architecture of bank interlinkages.