Tunneling the East River
The East River flowed over a mix of gravel, clay, sand, silt and bedrock. When sandhogs hit pockets of sand and gravel, there was a risk of air leaking from the tunnels. Workers were constantly on the alert for a whistling sound—a sign that compressed air was escaping and might at any moment punch a hole through the riverbed, known as a blowout. These blowouts were costly and often deadly.