Limestone forms at the bottom of the sea, where shell pieces settle in layers, get pressed together and stick.

Over a very long time, more and more rock piles up on top, so the limestone lies deep underground.

The limestone, deep underground, with rock piled on top.

Deep underground, two things happen to the limestone.

First, the rock above presses in on the limestone from every side, so the limestone is squeezed.

Second, deep underground it is hot, so the limestone gets very hot.

Squeezing from every side, and heat from below.

The heat and the squeezing change the limestone's grains.

The shell pieces turn into new grains, and the new grains grow and lock together.

The limestone has become marble.

Before: limestone, shell pieces in flat layers. After: marble, sparkling grains with gray wavy stripes.