Buried? In the ground outdoors?
Nearby lightning strikes can induce staggeringly large currents in the cable. Make sure there's a solid ground - like a copper stake - near the entry point into any building. Otherwise the current can travel into the building via the wire and set fire to things inside.
Even a not-very-close strike can destroy your Arduinos. For this kind of distance, I would always use MAX232 chips with some ESD (electrostatic discharge) protection. 15KV protection is really easy to do and protects you against all sorts of mishaps.
How are you powering the Arduinos at each end of this cable?