Something to me strange happended to today. Last night my sourdough rise to about double it's original size and this morning it was down to it's original size again. Why do you think it sank?
Are you talking starter? It does that. The rise is caused by gas generated as the started eats it's food. The bubbles reach the top and pop. The amount of gas produced fades as the starter uses up it's food. When no more gas is being produced the starter collapes.