https://phys.org/news/2018-11-extremely-large-hole-dug-telescope.html

Construction of the ELT began in May of 2017 and is currently scheduled to be finished by 2024. In the past, the ESO has indicated that it will cost around 1 billion Euros ($1.12 billion) to build the ELT – based on 2012 prices. Adjusted for inflation, that works out to $1.23 billion in 2018, and roughly $1.47 billion (assuming an inflation rate of 3%) by 2024.

In addition to the high-altitude conditions necessary for effective astronomy, where atmospheric interference is low and there is no light-pollution, the ESO needed a huge, flat space to lay the ELT's foundations. Since such a location did not exist, the ESO built one by flattening the top of the Cerro Armazones mountain in Chile. As the image at top shows, the site is now covered by a string of foundations.