Chartreuse, a French liqueur made by monks, is said to contain more than 130 herbs. It's available in most liquor stores.
Stir together Chartreuse, gin, lime juice, and simple syrup in a punch bowl. Pour in seltzer just before serving. Ladle into ice-filled glasses, and garnish each with a lime slice.
Punch can be refrigerated overnight. Add seltzer just before serving.