bottom of one's heart; depths of one's mind; innermost thoughts; real intentions
from the bottom of one's heart; wholeheartedly; truly; sincerely
心底から感謝している。
I'm grateful from the bottom of my heart.
その言葉に心底傷ついた。
Those words hurt me to the core.
心底惚れ込んだ相手と結婚できた。
I was able to marry someone I was deeply in love with.