Half-Life 2 Walkthrough -> Sand Traps
You'll soon come across a locked gate. To open it, you need to connect three batteries to the circuit in the red house next door. Two are there, and one needs to be pulled out from under the hood of the car in the yard (or from under the bathtub outside).
Drive the buggy into the garage, and the landing will begin. Run to the lighthouse, climb up, and shoot any ash-bombs in sight with a bazooka.
Look from the top of the lighthouse at the buildings below. You'll see a black dot in the rock to the right—that's the secret passage we're heading for now. As soon as the messenger comes running for you, descend into the lighthouse basement, where he'll open the door.
"Feropods"
After a short walk along the shore, you'll see a giant beetle emerge from the sand, not unlike this pot-bellied little thing. Lure it to the machine gunner next to the white door and pray you're lucky enough to outrun this beast until it dies (stand on the logs and fire in duplex mode with the shotgun):
Only after this will the door open. A friendly alien will perform strange manipulations on the beetle carcass, and you'll receive a new weapon – a pheropod (a beetle bait in the form of a small ball that fits easily in the palm of your hand).
Run after the alien, he'll teach you how to use this bait. Funnily enough, you can even throw it at people, forcing the beetles to chase them. Near the two logs blocking the passage, squeeze the bait a few times in your hand; this will lure the antlions toward you. The logs will lift, and you can continue your trek.
Near the boat, knock down the supports near the wooden structure so you can climb up to the ledge. Then, climb up the ladder by the cliff next to the pipes, jump onto the cliff on the right, and climb into the pipe at the end. Run past the guard towers, circle the area on the left, and you'll see the sign "Nova Prospekt."
I wonder what language that's in? Shoot down two cuttlefish, then turn right from the sign and climb through the hole in the wall. Turn the valves twice to put out the fire, and then through the door. End of Chapter 8. Next up is Nova Prospekt.