Porous - (of a rock or other material) having minute spaces or holes through which liquid or air may pass. 

Porosity - The quality of being porous, or full of tiny holes. Stems from the greek word, poros, meaning 'pore', which means passage. 

Passage - The act or process of moving through, under, over, or past something.     Journey. 

Threshold - A point of entry or beginning.