In recent months, there has been a welter of speculation about how many toxic assets are sitting in eurozone banks. Now, however, investors face another challenge: working out how many good assets those banks hold on their balance sheets.

The reason? As the Eurozone woes worsen, some politicians hope that the European Central Bank might support the banks again via another long-term repo operation (the process by which banks receive money by offering their assets as security.) But there is a catch: even if the ECB does another LTRO, this will only work if the banks have decent collateral to swap for funds. And some observers fear they are running low.

Last month, for example, Ray Dalio, head of Bridgewater Capital, the world’s largest hedge fund, warned his clients that “Spanish banks’ collateral is running out”. Like a cash-strapped household, which has pawned all its jewellery, these banks have already pledged away their valuable assets, he claims. Similar concerns are being muttered about other periphery banks. And though it is difficult to know whether such fears are justified, since public data are thin, investors would do well to watch the issue closely; and not just for the sake of the eurozone, but also because it casts the spotlight on a much bigger issue of collateral across the banking world.