Systemic financial risk is hard to see and measure, costing society greatly. The risk comes from how banks are connected through different financial contracts. By looking at layers of the Mexican banking system, we found that focusing on just one layer can underestimate total risk by up to 90%. Systemic risk levels of individual banks show the Mexican banking system has higher expected losses now than before the 2007-2008 financial crisis. Individual transactions can have much higher systemic risk than credit risk, posing big risks to the public. The total systemic risk of all layers combined underestimates the actual risk. This study is the first to objectively measure national systemic risk levels.