A mountain top.
I would think Powder Mountain Road SR158 going from Eden to Powder Mountain Ski Resort 16% grade for 7 miles.
The coast of Big Island of Hawaii has the steepest region of the Ocean Floor. If measured from sea bottom to mountain peak, it would be taller than Mount Everest. Rising up to Mauna Kea.
Steepest is an adjective.
The terrain is the steepest when it's on the left side of the map.
Scenic World's railway line is the steepest
Steepest Road in the UK, Ffordd Penllech, Harlech
Steepest line possible dealing with slope is up to, but not including infinity.
Pico Mountain is a best place to learn and perform ski from pinnacle to the steepest trail. It also offers tree skiing on Doozie and Brich woods. Among other options would be range of fun activities for all ages.
Takabisha is the world's steepest roller coaster located at Fuji-Q Highland in Japan.
Baldwin Street in New Zealand holds the Guinness World Record for the steepest street. It has a gradient of 35% at its steepest point, making it quite a challenging climb for visitors.
Your mother's line