Leaking cracks in basement walls are a fairly common problem. If there is no drain tile outside the foundation, fixing the crack may develop into a frustrating experience. A simple and effective method of sealing off the leak is with the use of bentonite. This is an old method, but not widely known in the building and residential construction fields. As with any material, the bentonite must be properly handled to get results. Bentonite is a pulverized rock which has been mined in Wyoming or South Dakota. It absorbs large quantities of water swelling to 30 times its original absolute volume, forming a jelly-like mass. When even only reasonably well contained, it prevents the passage of water. In new construction location of the shrinkage cracks can be predetermined by construction of control joints spaced every 20 feet. If the wall is to resist water pressure, the cracks can be waterproofed at the time of original construction. In old construction, bentonite can be used in several ways to stop leaks. A thin, dry layer can be installed on the outside of the wall. Another method is to plaster the wall with a bentonite gel. Still another method, is to veneer the wall with quarter inch thick corrugated cardboard sheets which are packed with bentonite.