The robot mole project at Monash University aims to develop robotic systems for locating the source of volatile chemicals released underground. As part of this project, a mobile robot, MOLE I has been designed that takes readings of underground chemical concentrations by inserting a probe into the ground at a sequence of locations. The robot is guided towards regions of higher chemical concentration using the hex-path algorithm. This algorithm was developed especially for the task of locating underground chemical sources and this is the first time that it has been tested using a mobile robot. There are important economic, environmental and humanitarian applications for systems that can find the source of an odor or volatile chemical located beneath the ground. These applications range from harvesting truffles, to finding the location of leaking chemical storage tanks and unexploded ordinance. In this paper, the robot construction and hex-path algorithm are described together with experimental results that demonstrate the capabilities of the robot system.