Halin's Mining Expedition
Halin’s mining expedition took place 10 years ago when a Dwarf from Erebor name Halin lead a small group of miners to Khazad-dum to explore and recover mineral wealth. They did not descended into the Hall of Trees, but instead veered south to take a secret passage to the lower mines. They managed to stay safe by collapsing a number of tunnels to prevent orc raids.
They managed to descend to the Fourth Deep and discovered a gold mine which they exploited for a few weeks. In the end, they were beginning to starve and fell prey to Moria Melancholy and had to pull out, collapsing more tunnels to cut any pursuits.
Apparently, the secret entrance to the mines remained a secret and no orcs ever attempted to cut their retreat through the way that they came in.