How reliable are public debt statistics This paper quantifies the magnitude, characteristics, and timing of hidden debt by tracking ex post data revisions across a comprehensive new database of more than 50 vintages of World Bank debt statistics. In a sample of debt data covering 146 countries and 53 years, the paper establishes three new stylized facts: (i) debt statistics are systematically under-reported
(ii) hidden debt accumulates in boom years and tends to be revealed in bad times, often during IMF programs and sovereign defaults
and (iii) in debt restructurings, higher hidden debt is associated with larger creditor losses. The novel data is used to numerically discipline a quantitative sovereign debt model with hidden debt accumulation and an endogenous monitoring decision that triggers revelations. Model simulations show that hidden debt has adverse effects on default risk, debt-carrying capacity and asset prices and is therefore welfare detrimental.