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You've decided to gear up with an environmental protection suit, something you can get from your work used in work like insulation or fungus infestations, which people at your work all collectively call the "Breaking Bad suit". It's not quite a spacesuit fitting for science, but it's a lot better then just going in with plain clothes, especially with a particle mask to filter out anything nasty just in case. You also bring along a selection of power tools with you, all borrowed from your work once again, which may be helpful if the inside of the tunnel is in any way similar to the inside of the rest of the building it spawned from.
You're now ready. You don the suit and bring along plenty of cameras, an extra bottle of water, and prepare for your first expedition.
"Well, youtube, it's time to see what lies within this place inside my place. The secret tunnel..."