The Bureau of Mines is investigating possible applications of geophysics to monitor leachate during in situ mining. Tomographic reconstruction of seismic or electromagnetic crosshole data appears applicable. The tomographic program BoMtom (Bureau of Mines tomography) was written and then used to investigate reconstructions with synthetic travel time data. The calculations confirmed that without constraints, crosshole data do not provide unique solutions. The solution is not determined solely by the data, but is also influenced by the velocity guesses starting the iterative solution procedure. The effectiveness of constraints in combatting nonuniqueness was examined. The requirement that the velocity not vary laterally at the top and bottom of the pixel grid was effective. This constraint is realistic when the top and bottom of the grid represent dry and saturated regions, respectively. Limiting the maximum velocity improved reconstructions, but may not be applicable in practice. BoMtom uses the simultaneous iterative reconstruction technique with straight ray paths. The program is efficient in use of memory, and can be run on a personal computer. A source code in Fortran is listed. A source code in basica and compiled executable codes in both languages are available from the Bureau's Twin Cities Research Center.