See LIGO Hanford Observatory Home Page.
I think LHO is quite happy to have visitors, but please keep in mind that we are in the middle of the S4 science run for the next two weeks, so the staff have their hands quite full and there is some need to keep the site as quiet as possible.
If possible, I suggest you postpone your visit until after the S4 run is over!
[Last week of March].
Cheers,
Bruce

Visiting LIGO Facility
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Please do come by an visit the lab, especially after the S4 science run as Bruce notes. We have a regular tour of the facility, each 2nd Saturday of the month at 1:30pm. Furthermore, in honour of the World Year of Physics and the anniversary of Einstein's 1905 annus mirabilus, we are hosting a series of lectures, films, and astronomical observations, please join us:
http://www.ligo-wa.caltech.edu/world_year/iyp_events05.html