Why does the Falgu River flow underground at Gaya?

According to the Ramayana legend, the Falgu River was cursed by Mata Sita to flow underground because it falsely denied witnessing her performance of Pind Daan for King Dasharatha. Since then, the river flows primarily beneath its sandy bed — an Antahsalila (underground river) — and pilgrims must dig into the sand to access its sacred water for Tarpan and Pinda preparations. Despite this curse, the river remains supremely sacred and indispensable for ancestral rites at Gaya.