(KTXL) — Since 1935, the Tower Bridge has served as the gateway to the capital city and the foothills of the Sierra Nevada, but today it also serves as an example of a rare bridge lifting system in California.

The Tower Bridge uses a vertical lift system to allow taller boats and ships to pass under the iconic bridge, but since 1935 this type of lift system has become rare to find in the Golden State.

According to Caltrans, there are only five remaining vertical lift bridges in the entire state.

What is a Vertical Lift and How Does it Work?

A lift bridge has two towers where a central span can be raised to allow for waterway passage.

The Tower Bridge has a central lift span of 209 feet, weighing around 2.3 million pounds, supported by the two 160 feet tall vertical towers. At its maximum height, the central span will be 100 feet above the water.

“To avoid the necessity of lifting this great weight as a direct load, it is counterweighted by steel frames, filled with concrete,” California Highways and Public Works wrote in a January 1936 report.

Each of the towers has a concrete-filled steel frame inside that runs in the opposite direction of the lift. Each counterweight is attached by 96 steel wire ropes to the four corners of the lift span.

The total weight of all moving parts to lift the central span is around five million pounds and is assisted by two small 100-horsepower electric motors.

When Does it Go Up and What is the Procedure?

When a vessel is looking for passage under the bridge, they send a signal to the operator room which sends a signal back to the vessel.

At that time sirens at each end of the bridge are sounded, stop signals are used at street intersections and illuminated signs warn vehicles the span is going to move.

When the bridge had a rail line running through the center of it, automatic signals would be sent to stop all approaching trains and all of the rail locks are unlocked.

The bridge can fully raise in one minute and 30 seconds and it takes the same time to lower it.