Complex sentence: Wet or dry, it can be a major fire hazard; peat fires may burn for great lengths of time, or even burn underground and reignite after the winter if an oxygen source is present.
Split into simpler sentences: Wet or dry, it can be a major fire hazard; it can fuel peat fires for great lengths of time. Peat fires can even burn underground and reignite after the winter if an oxygen source is present.