Some sections have been hollowed out over time, and are sagging 4. Grass and tree roots that have infiltrated 5 gaps pose another major threat to their stability.

随着时间的流逝，部分城墙出现空心区、正在塌陷。长在城墙缝隙之间的草、树根也对墙体的稳固构成威胁。

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It is true, they say, that tree roots can trap sliding soil but only for shallow landslides of up to a metre in depth.

ECONOMIST: Deforestation and floods

Today, it is blissfully quiet - though back in town, on the elegant Rue St-Louis, there is a cannonball still wedged in the roots of a tree that is said to date from that battle.

BBC: Qu��bec's wood runners

New York has experienced only 2.06 inches of rain the past 20 days after a winter that set records for snow, which funneled water to tree roots.

WSJ: Sniff, Sneeze, Blow, Wipe, Repeat