Turn the wheel labeled 'Flow Control' on the side of the silo all the way, climb the ladder up to the top as quickly as possible, and jump in.
Unless you are skilled at taking out headcrabs without taking damage, avoid them and run straight across the opening to the silo. The tunnel dumps you out into the bright sunshine, surrounded by ponds of goo.
Floating pieces of who-knows-what in a vat of delicious green goo.
