the.com/water table

means The upper level of underground rock or soil that is permanently saturated with water — dig below it and you hit groundwater.

from A 19th-century technical compound: 'table' here doesn't mean furniture but a flat, level surface — the same sense found in 'tableland' or a drafting 'table' of figures laid out flat. Hydrologists and well-diggers borrowed that leveling sense to describe the roughly horizontal boundary underground where rock or soil goes from damp to drenched. No deeper mystery than that — just geology needing a word for 'the line where it's wet,' and reaching for a surface already associated with flatness.