Secret Avengers #16

Run the mission! Don't get seen! Save the world! A secret city buried one mile under American soil is discovered through the leakage of Von Doom radiation – a type of energy emitted only by time-travel devices. So the Secret Avengers head underground to a weird metropolis forgotten about for decades, because a time machine in the wrong hands is the worst kind of WMD imaginable.

Characters: Beast, Black Widow (Natasha Romanova), Captain America, Moon Knight