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We have developed a novel planetary subsurface explorer that is capable of excavating lunar soil and carrying out scientific investigations. Our developed device consists of two units: a propulsion unit and an excavation unit. The propulsion unit that is based on the peristaltic crawling of earthworm maintains the body position and orientation of the robot and also reduces friction, which is the factor that traditionally prevents robots from excavating to significant depths. The excavation unit excavates and clears a space for the robot to tunnel into densely packed soil. In this paper, we discuss strategies for underground excavation. Next, we develop the excavation and propulsion units, and conduct several experiments to test these units. Finally, we develop a prototype subsurface robot with both units integrated in one package. The prototype exhibits good excavation performance in terms of depth reached-430 mm-both under its own full weight and for 1/6 of its own weight. In other words, the prototype shows excellent robustness to the gravity differences on the Earth and the Moon. With appropriate dust removal, operation has been demonstrated to a depth of 650 mm without any slowing down. The same performance is considered possible for much greater depths.