I'm not proposing this as an actual solution, it's just a dumb idea. But if we dug a huge, wide hole at the bottom of the ocean, or maybe widened the Mariana Trench or something, could that extra space make the sea levels drop enough to keep the land from flooding?

A majority of water in the planet (magnitudes larger than all the surface oceans) is within the earth so digging a whole would not fix anything. Besides the sea levels aren't rising.

Wrong. Now, shut up.

Peons never surprise me with their lack of understanding https://iere.org/where-is-most-of-the-water-on-earth-found/

Research and research some more on other sites, publications, scientific papers. More than 5 times of the earths water is underground. Also you've never saw anyone step outside and say "oh no, the water is up to my shoes now because the sea levels are rising" this even goes for people who live on beachfront properties.