The initial "th" is pronounced as a voiced dental fricative, that is, the
tip of the tongue is placed against the back of the upper front teeth and
air is allowed to pass through the oral cavity while the vocal cords
vibrate. The medial "a" is pronounced as a voiced lower front vowel, that
is, with the mouth relatively open and the tongue placed downward and
forward. The final "t" is a voiceless alveolar stop, that is, with the tip
of the tongue placed against the ridge just behind the front teeth and
sharply withdrawn while the vocal cords are silent. And that's that.

Andrew K. Rindsberg
Geological Survey of Alabama