Your sniffer has indicated the problem and now it's time to take action before the mildew runs rampant and you end up with a $10,000 hardwood floor replacement. Mildew thrives in moist dark areas, so to kill it you must brighten things up and dry them out. Prune back the spring growth outside your windows, pull up the blinds open the curtains and drapes, and flood your house with light. Prune shrubs away from foundation vents, and start a fan or three going under the floor and as many above it as well. The fast-moving air above and below your floor will dry out those impossible-to-reach places where only vapors travel. Keep this up all day every day for several days after the smell has disappeared.