Being forsaken by her husband, Sītādevī entrusted her two sons to the care of Vālmīki Muni. Then, meditating upon the lotus feet of Lord Rāmacandra, she entered into the earth.Śrīmad-Bhāgavatam 9.11.15-17
munau nikṣipya tanayau
sītā bhartrā vivāsitā
vivaraṁ praviveśa ha
munau — unto the great sage Vālmīki; nikṣipya — giving in charge; tanayau — the two sons Lava and Kuśa; sītā — mother Sītādevī; bhartrā — by her husband; vivāsitā — banished; dhyāyantī — meditating upon; rāma-caraṇau — the lotus feet of Lord Rāmacandra; vivaram — within the earth; praviveśa — she entered; ha — indeed.
It was impossible for Sītādevī to live in separation from Lord Rāmacandra. Therefore, after entrusting her two sons to the care of Vālmīki Muni, she entered into the earth.