In 1986, Congress enacted the Harbor Maintenance Tax (HMT) to recover operation and maintenance (O&M) costs at U.S. coastal and Great Lakes harbors from maritime shippers. O&M is mostly the dredging of harbor channels to their authorized depths and widths. The tax is levied on importers and domestic shippers using coastal or Great Lakes ports. Due to a Supreme Court decision in 1998, exporters no longer pay the tax because it was found unconstitutional. The tax is assessed at a rate of 0.125% of cargo value ($1.25 per $1,000 in cargo value).