The flooded mine

An original fiction passage set in the nineteenth century.

As Nell moved deeper into the old mine, she pressed an arrow into the clay at each fork. When she reached another division, she looked back. Water crept over the floor behind her and loosened the clay; the nearest arrow sagged into a shapeless groove. Beyond it, the three openings she had already passed looked identical. In front of her, one passage rose steadily, and her lantern showed that its floor remained dry. Nell paused, then continued towards the higher ground.

Give two reasons why continuing forwards may be safer for Nell than turning back.