Phytoremediation, a newer green technology, is the use of green plants to remove pollutants from the soil or render them harmless. Certain plants, such as alpine penny-cress (Thlaspi caerulescens), have been identified as metal hyperaccumulators. Researchers have found that these plants can grow in soils contaminated with toxic heavy metals, including cadmium, zinc, and nickel. The plants extract the toxic heavy metals from the soil and concentrate them in the stems, shoots, and leaves. These plant tissues may then be collected and disposed of in a hazardous waste landfill. Researchers are investigating ways to recover the metals by extracting them from the plants. Although phytoremediation is limited by how deep the roots of the plants grow in the soil, it is a less costly alternative way to clean up hazardous waste sites.