expression
staring piercingly; looking hard
Used with verbs like 見る or 眺める to describe staring so intently it feels like boring a hole through something.
彼は穴の開くほど私の顔を見つめた。
He stared at my face so hard it felt like he was boring holes into it.
Literally 'enough to open a hole'; an idiomatic phrase emphasizing the intensity of a gaze.