シン, ふか.い, ぶか.い, ふか.まる, ふか.める, み
deep, heighten, intensify, strengthen
JLPT3 G3 S11 F484
As per the right-hand element# as described in 探 (deep) + 水 water → *deep* waters → *profound*; *intense*. The original reference may have been to a pool/lake located deep within the earth.
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