Strengthened glass strong enough to create underwater rooms - and stop smashing in storms.
I've always built over water, don't know why. And LOVE the idea of having steps to an underwater section with glass that that allows you to have a viewing area. Lights that work underwater would be awesome too. Strengthened glass walls would also stop smashing in above ground bases. Just getting a bit tired of whack a mole with my hammer every time there's a storm!
If you hook your glass bench up to a powered water pump, the glass made will go from 500 to 2500 durability, the same as basic stone. Underwater would be cool, but with 2500, glass does good against most stuff (except falling trees).