Context: Satan and his fellow rebel angels, having been thrust into Hell by God's power, prepare to build themselves a city and a glorious center of government. They dig, at the suggestion of Mammon, into a hill which appears to have minerals within it. The spirits soon have opened "a spacious wound" in the hillside and dig out ribs of gold to use as material for building. Milton, well guessing the reaction of his readers, comments, using the original meaning of "admire": "to wonder."
Let none admireThat riches grow in hell; that soil may bestDeserve the precious bane.