mundus, according to Plutarch, a pit in Rome dug by Romulus (traditionally sited in the Comitium) in which he put first fruits and earth from each country from which his followers came, afterwards filling it up and putting an alter on it. In other Italian cities it was the name given to a pit dug to give access to the manes (of the Underworld). The Roman mundus was closed with stone except for three days of ill omen, 24 August, 5 October, and 8 November, when mundis patet, ‘the pit is open’. This pit seems to have been called mundus Cereris or Cerealis, ‘pit of Ceres’.
– The Concise Oxford Companion to Classical Literature