Catacombes of Odessa. Odessa is located on land that millions of years ago was covered by the Black Sea. Shells of mollusks combined and formed light yellow shell rock. Examining any of this rock, a person can see that it is composed of millions of shells. Sandstone was both the foundation for the city and a primary building material. Easy to dig through, the sandstone allowed for the construction an estimated 2,000 kilometers of labyrinths stretching out under the city. Nearly the entire older section of Odessa is built with this stone.