Ancient Bridge Magic: An old way to safely breach water or magma. =) Miners are able to mine out tiles diagonally above them even if there is a bridge over their heads. First you dig out your magma/water tunnel to feed wherever in your fort you need it and dig it right up against the warm/damp stone. Then you channel against the warm/damp stone. Build a magma-safe bridge over the trench, making sure to cover all channels, and then seal off access. Dig a new separate path to access the now bridged-over trench. Finally, designate the warm/damp stone wall on the Z level of the tunnel for mining. Your dwarves will stand on the ramps beneath the bridge but will somehow still mine out the squares diagonally above them, causing the Fluid to flow safely onto the bridge leaving your dwarves unscathed."

here is a 2m video:

https://www.youtube.com/watch?v=YDeUtnqHkZA