PEABODY – A massive brush fire began burning underground Sunday and continued to blaze a trail into Monday night.Deputy Fire Chief Paul Lynch estimated that 15-25 acres of brush were ablaze, but said the fast-moving fire was not threatening any homes or businesses in the area.”There is a definite glow in the sky, no question about it,” he said. “This (brush fires) used to be an annual thing with us, but it hasn’t burned in years, so with the thickness of the brush, the fire gets super hot and, with the wind, the fire feels like a blow torch.”By nightfall, Lynch said the fire line had stretched from about a quarter-mile to a half-mile long, from Meadow Lane to Sunset Road.”It’s a slow burn underneath, but we managed to stop it from going onto the golf course (The Meadow at Peabody),” he said. “The guys were there from 7 a.m. to 5 p.m., they’re beat and you have to worry about them getting hurt. They can’t do any more, but they’ll be back (today).”A State Police helicopter monitored the area during the afternoon, but by Monday night, Lynch said he was stumped as to how the fire was sparked.Lynch said Lynn Engine 9 responded to help battle the blaze, while Lynnfield, Beverly and Swampscott supplied mutual aid.