Bond issues for PPP projects typically were 'wrapped' (i.e. interest and principal payments were insured) by specialist companies (monoline insurers), investors thus receiving the benefit of the monoline insurers' own AAA credit ratings. (PPP projects' own underlying ratings typically are low investment grade - BBB.) Most monoline insurers had heavy exposure to the US sub-prime mortgage markets, and the weakening of their balance sheets and associated credit rating downgrades meant that bond investors became unwilling to accept the risk of monoline insurance policies. In addition, bond underwriters became much more cautious, with several having been exposed with large volumes of unsold bonds on their books. Consequently, access to the capital markets became more difficult and, by the end of 2007, the capital markets were effectively shut to infrastructure projects. Although recently there have been some emerging signs of life in the capital markets and in the remaining monolines' insurance policies, the main source of finance for PPP projects since the end of 2007 has been banks.