בראשית, פרק ז׳, פסוק כ׳

The floodwaters reach their absolute peak, erasing the final refuges on earth and creating a single, unbroken surface of water. The specific height of the water is not merely a geographic detail but a practical necessity for survival.

The primary approach among commentators is that the recorded height of fifteen cubits does not refer to the water's total depth from the ground, as mountains are significantly taller. Instead, the water rose fifteen cubits above the summits of the highest mountains [רש״י, מזרחי, גור אריה, רד״ק, שפתי חכמים]. The flooding occurred in clear stages. First, the valleys and lowlands filled until the water level matched the mountain peaks. Only then did the waters surge upward by an additional fifteen cubits [רש״י, תורה תמימה]. This understanding points to a natural leveling of the water, dismissing the unlikely scenario where the water stood like rigid walls, artificially tracing the uneven shape of the land at a constant height over both valleys and mountains [גור אריה, תורה תמימה].

There is a highly specific reason for this exact measurement. The total height of the Ark was thirty cubits. By providing a cushion of water exactly fifteen cubits deep above the highest peaks, God ensured that even if the Ark sank into the water up to half its height, it could glide safely over the mountain tops without crashing into them [קאסוטו, חזקוני].

Noting again that the mountains were covered might initially seem repetitive, as their total submersion was already established in the narrative. To resolve this, some explain that the account is outlining a strict sequence: only after the mountains were completely submerged did the water rise the extra fifteen cubits [הכתב והקבלה, ברכת אשר]. Another perspective suggests that while earlier details simply stated the fact that the mountains were covered, this specific account reveals the exact depth and extent of that coverage [קאסוטו]. The immediate result of this absolute engulfment was the destruction of all living creatures, as every possible high ground vanished, leaving no dry place to escape [חזקוני].