Karst is a type of terrain characterized by hidden rivers and lakes. Typically the terrain consists of limestone, as this easily erodes, and the section of stone underneath the surface has been dissolved. Most cave systems in the world are karst areas. Likewise, there are "karstic" rivers, which can flow below the ground at points. If the soil is dense, the water will be on the surface; if porous, it will be underground.
The concept of karst can be transferred into the spiritual life. In fact, Fr. Raniero Cantalamessa has coined a term he calls “karstic prayer.” He writes: “At times, when we are free to pray, our prayer surfaces and becomes conscious prayer of praise, adoration. At other times, when our activities absorb us, prayer sinks to the bottom of our hearts and continues to flow hidden there, like an invisible movement of love toward God, ready to be aroused again as soon as we can do so” (Mary, Mirror of the Church, p.162).
When we clear our mind of thoughts and merely allow our heart to be open to Christ's love, we do not always know what is going on, and we do not always feel something. God is still present, like that underground river. And this is when prayer is most authentic and powerful. We are not praying for ourselves, for good feelings. We are praying purely out of love for God. There is an inner desire in us to be united to God, and this is what subtly moves us.
Mary is the great model of karst. She was hidden most of her life and still acts in an underground fashion today. She can assure us that there is indeed a river of love in us when we do not sense it.