Half-Life veteran quadrazid decided to give the classic FPS a whole new perspective and turned everything upside down. Headcrabs are a bit easier to spot this way, but besides that, things just get more difficult and confusing. Quadrazid's playthrough is still fast and smooth, though.
But it's not just hard when the ceiling becomes the floor. An upside down Black Mesa Research Facility is full of unexpected moments:
Here's the playthrough. This is the first part, so we'll have to wait a bit to see what happens when he gets out of the facility.