Four consecutive underwater explosions rocked a sleepy village on South Korea’s east coast on Monday, sending a spray of water onto the beach. Dozens of ROK marine amphibious assault vehicles (KAAVs) steamed toward shore, the roar of their engines growing louder as they prepared to land hundreds of troops. U.S. F-35B Lightning II fighter jets streaked overhead, providing air support.

The display of military might near the southeast city of Pohang was part of the annual U.S.-ROK Ssang Yong joint exercises, during which hundreds of marines from both sides trained to carry out an amphibious assault as reporters looked on.