Similar to dandelions and parsley, when dried out, crushed up cilantro absorbs metal from water, particularly lead and nickel.
Both dangerous metals to have present in drinking water, lead and nickel are absorbed through cell walls of cilantro in a unique display of biosorption. It can be used in tea bags, which are placed in a pot of water for three minutes and then removed, or by stuffing it in a tube and dripping water through it (similar to a Britta filter). The lead researcher of a study taking place outside of Mexico City, Douglas Schauer, noted that a "handful of cilantro will nearly cleanse a pitcher full of highly contaminated water of its lead content."
This has proved a successful filter in areas around the world where the activated carbon filters, like those manufactured by Brita, are expensive, particularly in regions of rural farming. People can go grab some from the ground when a new filter is needed.
More study is being done, including trying to find out if arsenic and mercury are absorbed by the herb as well and if there is some sort of "synergy" when multiple metals interact with the herb at once.