Can zombies attack downward?

In older versions of 7D2D (earlier October of last year) zombies couldn't attack/dig vertically down, and or break ground hatches even if they were present at the surface. The only way they should be able to "attack down" is if they were to get wedged or stuck in a hole somewhere around the hatch.

Similarly they can't attack up, so if for example you're suspended above iron bars over an open ground, they'll just crowd under you while you hover. Some factors you've got to keep in mind is of the feral horde's weight that may be on your hatch, as that could cause it to cave in.

To answer your question though,even if the hatch appears to be on the ground, it still is using that one block above ground where it is placed. So technically the hatches hit box is waist level and that's why they are able to hit it.

Just a quick tip, but digging at an angle until you hit bedrock is fairly effective if you can manage. By then, you'd be a long distance from your entrance. When a horde spawns, they want to go directly to you, where you're standing at. So yes, they'd be above you, but away from the hatch, which could prove to be useful.

Adding an answer now that none of the existing questions are correct due to gameplay changes.

Zombies can now (posted as of Alpha 17.4) attack downward. I'm not sure when exactly this went into effect, but they can now hit the ground or block that is directly underneath them. This includes, dirt, stone, hatches, and any other block. It has definitely changed the strategy I employ in dealing with the zombies, as their new AI has much more effective pathfinding and can get to you much easier.

I have had zombies dig underneath a wall by digging down one block then forward under the wall. They only dug down one block, so only crawlers could go through the gap, but they did it. Similarly, I have also had zombies dig through a bunch of sand after falling into a pit, then dig up through my floor. I don't know about older builds (only started playing recently) but it seems in current builds that yes, they CAN dig up and down.