Problem 1 - Entrance Test

Read the following passage: 'The chthonic murmur of the city, a palimpsest of forgotten dreams and aspirations, seeped into her bones. Each echoing footstep on the rain-slicked pavement was a metronome marking the passage of irrevocable time, a relentless cadence against the fragile architecture of her memories.' What is the most likely meaning of 'chthonic' in this context?