BANGKOK—As the massive undersea Hunga Tonga Hunga Ha’apai volcano erupted on Saturday, Tongans from around the world gazed on as their relatives live-streamed images of billowing clouds of ash, gas, and steam emerging from beneath the depths. Then darkness. The eruption severed Tonga’s single fiber-optic cable, rendering the entire Pacific archipelago offline, and unable to communicate with the rest of the world—leaving their loved ones terrified about what might have happened. “It was absolutely crazy,” said Koniseti Liutai, a Tongan who lives in Australia. “We were talking with family and relatives, because they were excitedly showing us the volcano’s activities, then we heard the explosion and the big bang and everything went dark,” he said. “Then the next information we got was the tsunami warning and then the tsunami hitting; we were all absolutely fearing the worst.” It wasn’t only family and friends who could not get through. Huge ash …