The following excerpt is from Kundalini Yoga: The Mother Power, by Sri Chinmoy.

When we think of the chakras, we think of them as being inside something – that is, inside the subtle body. Sthula sharira is the gross physical. Inside this body is the sukshma sharira, the subtle body. And inside the subtle body is karana sharira, the causal body. These bodies are totally different, but they are all members of the same family. The subtle body at times operates through the physical. At times the causal body, which is in seed form, can operate in and through the subtle body and the physical body. Although the causal body is like a tiny seed, from this seed a banyan tree can grow. It takes a few years, but then the banyan tree bears thousands of fruits. The potentiality of the seed is infinite.

Now, the chakras are not only in the subtle physical; they are also in the universe. According to some Masters, the spiritual heart is the universe; they have gone so far as to say that the universe is inside the heart. Nothing is beyond or outside the spiritual heart.

Writings on Kundalini