noun, used as a suffix
... or deeper; ... and below
Suffix attached to a depth measurement to indicate 'at or deeper than' that depth. Used in technical or formal contexts such as diving, drilling, or geological descriptions.
水深50メートル以深の海域では、特別な潜水装備が必要です。
In waters 50 meters or deeper, special diving equipment is required.
この地層は100メートル以深に存在する。
This stratum exists at depths of 100 meters and below.
Compound of 以 (from, beyond) and 深 (deep). The exact historical derivation is uncertain, but it follows the pattern of other 以- compounds like 以上 and 以下.