US Navy sailors repair damaged port walls at Palau during a training event in 2024.Chief Builder Tyler Ault/US NavyDARPA wants to 3D print concrete underwater using seafloor sediment and seawater.The supporting tech could autonomously repair ports, seawalls, cables, and other maritime infrastructure.The military sees underwater construction as a potential advantage in contested logistics.The US military's research arm wants to find ways to 3D pr…