I forgot to add, the reason they don't drill down is because it's hard and expensive. The arms are 4km, and to get the required sensitivity they bounce the light about 300 times before detecting it.
Drilling down 4km would be quite expensive, especially since you need a large mirror assembly[1] and vibration isolation system[2] down there. The mirror assembly and vibration isolation systems would also have to be of a different design, further increasing cost and complexity.
Drilling down 4km would be quite expensive, especially since you need a large mirror assembly[1] and vibration isolation system[2] down there. The mirror assembly and vibration isolation systems would also have to be of a different design, further increasing cost and complexity.
[1]: https://twitter.com/ligo/status/601848954645401601
[2]: https://www.ligo.caltech.edu/page/vibration-isolation