The earth could have been "smoother" than it is today, the ocean floors not as deep as they are today letting the water available back then cover the earth
That'd be tough. The Earth is already smoother than a billiard ball.
quote:
The World Pool-Billiard Association Tournament Table and Equipment Specifications (November 2001) state: "All balls must be composed of cast phenolic resin plastic and measure 2 (+.005) inches [5.715 cm (+ .127 mm)] in diameter and weigh 5 to 6 oz [156 to 170 gms]." (Specification 16.)
This means that balls with a diamenter of 2.25 inches cannot have any imperfections (bumps or dents) greater than 0.005 inches. In other words, the bump or dent to diameter ratio cannot exceed 0.005/2.25 = 0.0022222
The Earth's diameter is approximately 12,756.2 kilometres or 12,756,200 metres.
12,756,200 x 0.0022222 = 28,347.111
So, if a billiard ball were enlarged to the size of Earth, the maximum allowable bump (mountain) or dent (trench) would be 28,347 metres.
Earth's highest mountain, Mount Everest, is only 8,848 metres above sea level. Earth's deepest trench, the Mariana Trench, is only about 11 kilometres below sea level.
So if the Earth were scaled down to the size of a billiard ball, all its mountains and trenches would fall well within the WPA's specifications for smoothness.
copied from here