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Elpipes are polymer-insulated underground HVDC power lines that use relatively rigid extruded conductors designed for higher capacity and efficiency than is practical for overhead power lines. Elpipes can use far more conductor than cables, but also have more splices that an HVDC cable. The US Department of Energy showed early interest in massive underground transmission, but that faded until recently. The high efficiency of elpipes is motivated by the need to minimize heat dissipation while maintaining passive cooling. Minimizing waste heat production is critical since heat dissipation limits capacity. For a 325-800kV DC elpipe, we have selected a design basis of 1% loss per 1000 km, about three times better than an overhead 800kVDC line, and similar to “high temperature” superconducting (HTS) lines after accounting for the energy HTS lines consume for cryogenic cooling. This high efficiency enables continental scale power transfers with acceptable loss, and fully buried elpipes carrying up to 12 GW. Elpipes may one day enable continental scale HVDC grids, though there remain technological hurdles such as the need for HVDC breakers that must first be solved before that can occur.