ET will be the successor of the LIGO/Virgo telescopes, built underground and much bigger in size. Hosting the telescope has obvious consequences for local employment and national prestige. The two main contenders as of now are Italy and the Netherlands.

I will resolve the question to "in Sardinia, Italy" if by the end of 2027 there is consensus that a decision has been reached to build the telescope there. Similarly for "near Maastricht, the Netherlands".

If a different site has conclusively been chosen that is more than 250 km away from Maastricht and is not located in Sardinia (as defined by the administrative borders of the region) I will resolve to Other.

If by the end of 2027 the telescope location will still not have been chosen, I will resolve to "it will not be decided by the end of 2027".

If both sites will be chosen and two telescopes will be built, I will resolve to "two telescopes will be built in both locations". Note that these must be two actual telescopes that will take gravitational wave data and that will be bigger than current state of the art interferometers (e.g. Virgo), not just administrative offices or data centers or prototypes. If, for instance, a telescope is built in Sardinia and a related facility (that is not a telescope) is built near Maastricht, I will resolve to "in Sardinia, Italy".

• Update 2024-12-12 (PST): If one telescope is built in Sardinia and another telescope is built at a location that is not within 250 km of Maastricht, the market will resolve to both Sardinia and Other. (AI summary of creator comment)