How to reach the planets core in Astroneer?

In my experience, you have to dig downward to reach the center, which is called a Gateway Engine (so yes, they are entombed). You can't find them by just exploring caves. In theory, you should be able to dig straight down and discover it but I wouldn't recommend this since you will almost certainly hit a cavern along the way and fall to death (not to mention likely suffocate).

I've notice that once you get closer to the center of the planet, gravity will lessen, allowing you to jump higher. All audio effects will also sound a lot more echoey. When you begin to notice these signs, you should be getting fairly close, so just keep digging downward.

I've found the best strategy is to dig at roughly a 45 degree angle downward. You could go steeper, but it may be hard to climb your way back up eventually.