The tidal bore (or large wave) on the River Severn in England is famous, but fewer people know of the Trent Aegir, the tidal bore on the River Trent that causes a wave that can rise up to five feet high and flows upstream. It’s one of England’s most interesting and unusual natural phenomena.

Impressive bores like the Trent Aegir naturally occur on rivers with large tidal ranges. The best tidal bores occur when low river levels, such as during a drought, coincide with spring tides (though high spring tides during normal summer months will still often produce a good display). The phenomenon is caused by a funneling effect brought on by the shape of the river: When high tides meet a shallow river with a narrow sea outlet, the flow will sometimes surge against the natural current, creating what’s essentially a wall of water that moves against the current.

When word gets around to expect impressive bores, the canal basin at West Stockwith, which provides a good vantage point, attracts large crowds at often unreasonably early times of the morning.

The story of King Cnut deliberately trying to turn the tide to show his subjects he was not infallible is said to have taken place in Gainsborough. It’s possible the Trent Aegir was what he was attempting to change. The name is said to be taken from the Norse god of the ocean. It is also known as the Eagre.