On both sides of the corridor's stone walls, there's a small carved groove every few distances, and these grooves aren't very high, almost within Zhou Feng's reach without extending his arm. The light illuminating this corridor comes from these grooves.
"These grooves contain a lot of lamp oil, which is refined from ancient corpse oil. It can be stored for millennia without rotting and won't dry out. It's extremely flammable, and most importantly, no matter how you burn it, it doesn't deplete even a little. So, this lamp lit by corpse oil is called the Long-burning Lamp!" Seeing Zhou Feng's confusion, Li Fangyuan explained, as he's an ancient demon who's been to many such relics, having seen many Long-burning Lamps, so he wasn't surprised.
However, Zhou Feng, being young, was naturally curious and amazed upon first seeing these Long-burning Lamps.
