A rat pit is one of those under-ground novelties occasionally seen in Boston by gaslight.

From Police records and recollections, or, Boston by daylight and gaslight for two hundred and forty years, by Edward Savage, 1873:

The pit consists of a board crib of octagon form in the center of the cellar, about eight feet in diameter ad three and one half feet high, tightly secured at the sides. On three sides of the cellar are rows of board seats, rising one above the other, for the accommodation of spectators. On the other side, stands the proprietor and his assistant and an empty flour barrel, only it is half full of live rats, which are kept in their prison-house by a wire netting over the top of the cask. The amphitheatre is lighted with oil lamps or candles, with a potato, a turnip, or an empty bottle for a candlestick. Spectators are admitted at twenty-five cents a head, and take their seats, when preparations for the evening’s entertainment commence. The proprietor carefully lifts the edge of the wire netting over the rat barrel, and with an instrument looking much like a pair of curling tongs, he begins fishing out his game, rat by rat, depositing each carefully inside the pit until the requisite number are pitted. The assistant has brought in the dog, Flora, a favorite ratter, which he is obliged to hold fast by the nape of the neck, so eager is she for the fray. Then commences the betting, which runs high or low according t the amount of funds in the hands of the sports

“A dollar. She kills twenty rats in twelve seconds!” “I take that!” “Half a dollar on the rats!” “Don’t put in them small rats!” “Two dollars on Flora in fifteen seconds!” “Done at fourteen!”  “No you don’t!” “Don’t put in all your big rats at once!” “Five dollars on the rats in ten seconds!” (no takers.)

The betting all seems to be well understood, but it would puzzle an outsider to tell whether there were really any genuine bets or not.

The bets having been arranged, time is called, and Flora is dropped into the ring. Flora evidently understands that her credit is at stake; but the growling and champing, and squealing, and scratching is soon over, and the twenty rats lie lifeless at the feet of the bloodthirsty Flora, when time  is again called, and the bets decided, and all hands go up and liquor. This exhibition is repeated several times, with different dogs, and lasts as long as the live rats hold out.

Source:

Savage, Edward. Police records and recollections, or, Boston by daylight and gaslight for two hundred and forty years. Boston: J.P. Dale, 1873.