the.com/mill pond

means A surface of water so smooth and unruffled it looks glassy still, often used to describe calm seas or calm situations ('calm as a mill pond').

from From the literal mill pond — the still, dammed-up reservoir of water kept behind a watermill to feed its wheel at a steady rate. Because the pond was enclosed and shallow compared to a running river, its surface stayed flat and glassy even while water was quietly being drawn off underneath to turn the mill. By the 1800s 'calm as a mill pond' was already standard English for deceptive or total stillness — the phrase borrowing the pond's placid face while knowing full well there was working current beneath it.