Rats! Tunneling into my dirt crawlspace!
make double darn sure that that dirt your seeing isnt a result of termites. They love to bring mud tunnels inside where they make runs. If it is loose soil,whats under it,the hole?? Dryer vents are notorious for allowing rats and mice inside. FIRST advice is use ONLY solid metal pipe,NOT flex plastic pipe. Once mice or rats are inside,they quickly chew thru it and they are inside your house. Secondly,the metal flapper kind is the worst for letting them in. A scrap of lint keeps door open a fraction and they can scurry in. The kind that rises 'up' when air is blowing out helps keep them out. gravity drops it back down. occasionally put your hand in there and wipe out lint rems.
If you have alot of mice,,,a revolving tin trap that repeats many times helps. Put popcorn in it for bait,cheese etc. the wind up like an old time alarm clock