A joint project between Russia, Germany, Poland, Slovakia and the Czech Republic, construction on Baikal-GVD started in 2015. The actual ‘telescope’ consists of hundreds of spherical modules made of glass and stainless steel connected to the surface through a set of cables. Those sensors currently occupy an area measuring 17,657 cubic feet. Over time, the plan is to add more sensors to make the telescope even bigger.
As to why they would put those modules underwater, it’s a useful medium for detecting neutrinos and Lake Baikal has a lot of it. Located in South Siberia, it’s one of the world’s largest and deepest freshwater lakes. It’s also pristine and covered by ice for at least two months of the year. There aren’t many places on the planet that are as ideal for this type of research as Lake Baikal. The only two other telescopes that match it in scale are the IceCube Neutrino Observatory in Antarctica and the ANTARES telescope deep underwater in the Mediterranean Sea.