Bunker under highly controlled-atmosphere to measure Earth’s rotation

Equipment with extremely stable laser requires perfect mastery of air pressure and temperature.
German scientists are building the most precise and advanced laser in the world in a deep underground bunker near the German-Czech border. This is the most stable ring laser ever and it will be used to make unprecedentedly accurate measurements of the Earth’s rotation.
But such extreme stability can only be achieved if the laser is protected from outside influences such as changes in air pressure and temperature, so the pressurized cabin where it is situated is 6 m underground behind a 20-m long tunnel and five cold storage doors.
The laser has started providing the first ever measurements of the Earth’s wobble, a migration of its axis with a radius of approx 9 m. It achieves this via two counter-rotating lasers travelling around closed beams paths. Any rotation of the apparatus causes a slight discrepancy between the distances both beams have to travel. The beams therefore adjust their wavelengths to compensate for this which in turn affects their optical frequencies.
By measuring these shifts in frequencies, the scientists can deduct the changes in rotational velocity the system experiences.