念念不忘只因刻骨铭心
[niàn niàn bù wàng zhĭ yīn kè gú míng xīn]
Roughly translated into ‘Unforgettable, only because deeply imprinted within heart’. Expresses a sentiment of unforgettability due to deep and intense emotion or memories. It means something unforgettable due to profound and moving experiences or feelings involved.