Sansevieria spread by underground rhizomes. I had one actually bent and eventually cracked the plastic pot I had it in. I understand that even clay pots are not totally safe from these plants' roots!
Sansevieria are best grown in deep containers, so the rhizomes have room to grow down without damaging the pot. Once they have grown back upward, emerged from the soil, and formed leaves, they can be separated from the parent plant and potted up individually. They can also be propagated by cutting a leaf crosswise into sections and planting them in soil.
The International Sansevieria Society has some interesting information on their website:
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams