According to the Bicolanos, the halo-halo started from the simple mongo con hielo—boiled mung beans, milk, sugar, and shaved ice—sold by Japanese-owned soda shops in Naga, Camarines Sur. During World War II, these innocuous soda fountains took on a sinister role, as Danny Gerona recounts in his book Naga: The Birth and Rebirth of a City:
Almost five years before the Japanese invasion of the Philippines, the province of Camarines Sur registered a substantial number of Japanese residents. There were about eight retail stores owned by Japanese, most if not all of these were in Naga. Among the more prominent Japanese establishments in Naga were the Filipino Bazar [sic] and the K Mori refreshment parlor, which served the most desired cold drink of mongo con hielo. The residents of Naga and their Bicolano customers had no idea about the mission of these Japanese merchants until the Japanese Army arrived in Naga. To the residents’ surprise, most of these Japanese merchants donned their military uniforms and turned out to be high-ranking officers in the Japanese Imperial Army.