CONCORD, N.H. (AP) -- Lawyers for ExxonMobil (NYSE:XOM) have begun presenting their defense against the state of New Hampshire's claims that the oil giant should pay hundreds of millions of dollars to clean up groundwater contamination from the gasoline additive MTBE.

Jurors returned Monday following a weeklong break after spending six weeks hearing the state's witnesses.

The first defense witness is a former ExxonMobil environmental specialist who warned of MTBE's environmental hazards in the mid-1980s. Former Department of Environmental Services Commissioner Robert Varney testified that he was "shocked" ExxonMobil did not share her findings with the state, but lawyers for ExxonMobil say the information was widely available at the time.

Lawyers for ExxonMobil also contend MTBE did what it was supposed to -- reduce air pollution.