I imagined it'd too impractical to bury them in case they need repairs, but apparently they're buried at least in some cases.

According to [1], a plough is dragged along the ocean floor behind the cable laying ship. The plough digs a trench that the cables are fed into. I can't really tell from the pictures whether the trench is covered back up.

[2] shows a really cool animation. Apparently ploughs like these can bury cables 2-3 meters into the ocean floor, and use a jet of water to help dig. Good stuff.

1: https://www.quora.com/How-are-major-undersea-cables-laid-in-... 2: https://www.youtube.com/watch?v=9hEDTRU_F2s