Heat oven to 375℉/190℃.
Toss carrots, celery, onion, mushrooms, tomatoes, corn cobs, and garlic with olive oil.
Spread on a baking sheet.
Roast for 30 minutes, stirring every 10 minutes.
Place vegetables in a 12 quart stock pot.
Add thyme, bay leaves, peppercorns, salt, and water.
Bring to a boil.
Reduce heat.
Cook until reduced by half, about 1 ½-2 hours.
Strain out vegetables.