JAKE SPRING THE world's largest wetland is ablaze, but the fire is often invisible. In Brazil's Pantanal, the vegetation compacted under the marshy floodwater during the wet season dries out as ponds and lagoons evaporate, leaving flammable deposits underground that can continue to smoulder long after visible flames die down. Firefighters across Brazil are battling raging towers of flames from the Amazon rainforest to the Cerrado savannah, but the fires beneath their feet are a particular challenge in the Pantanal. The only way to combat an underground fires is to dig a trench around it, said state firefighter Lieutenant Isaac…