The mishap occurred when the victims in their early 20s after attending a function at Rajapeta Cross Road in Karnataka were returning to Kuppam, police said.
In the wee hours, the youths riding two motorcycles did not see the trench being dug for constructing a culvert and fell into it, they said.
The injured has been admitted to a private hospital in Kuppam and his condition is stated to be critical, police added.
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