Seattle is known for tree-huggers, Earth-firsters, moss-between-the-toes nature lovers. So, here's a story about how Seattle got this way by leveling hills, filling valleys, and rerouting water.
If you think Seattle is hilly now, you should've seen it then - then being about 100 years ago. There were seven hills in Seattle, or at least folks were willing to creatively count them so they'd have a local version of the seven hills of Rome. They thought big back then.
Seattle is a city built on grids. It could've been one big set of rectangles with jagged edges along the shore, but instead, a mix of grids fell into place by chance (or debate) as much as by plan. The Denny Triangle is a relatively flat piece of ground bordered by Denny on the north, Stewart on the southeast, and water on the southwest. Look at a map and it is the triangular grid that connects the grid that's around Seattle Center to the grid that is most of Downtown. It must be some of the easiest ground to plan for, flat without being in a floodplain.
It didn't start that way. The hill was in the way of progress. Water cannons were regularly used for mining, so they brought some in and mined away the earth without looking for gold or minerals. Imagine what would've happened if they found something worth processing. But, like many hills around the Puget Sound, the water only had to cut through what was left over from the glaciers.
It took years of pumping Lake Union water up the hill to carve the hill away. After the first wave of work, they returned with steam shovels to finish the rest. The goal was to make flat land so that the city could grow, and they succeeded. Now, the Monorail runs above it, Belltown lives on it and Highway 99 runs under it.
The hill didn't vanish. It was just moved, downhill of course. You drive across it when you're down on the waterfront. The hill became the fill that redefined the area around the warehouses and piers.
As for the name, if Seattle was named after its (European) founders, it could just as likely have been called Denny. But, that's another story.