Can Specters move into the ground to avoid Opportunity Attacks?

Question

In 5e, Specters have the Incorporeal Movement trait, which says:

The specter can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.

Therefore, I assume Specters can move through ground as if it were difficult terrain.

Could a specter attack a creature, move into its space, and then move straight down into the ground to avoid an Opportunity Attack?
Regardless of whether it would be intelligent enough to do so, would the OA happen before it is fully submerged below ground? Or would the ground protect the Specter?

My guess would be that the ground would protect it, because once it leaves the 5-foot reach of its target, it would be mostly submerged.

Answer 1

No, they still provoke an opportunity attack.

You can make an opportunity attack when a hostile creature that you can see moves out of your reach.

You cannot reach into the ground, and the Specter entering the ground means it has left your reach, provoking an opportunity attack. Though Jeremy Crawford's tweets are not official Sage Advice, he has still agreed with this position (specifically about burrowing, though it applies the same with incorporeal movement):

Tweet 1:

Q: Does an earth elemental trigger an OA when it burrows? It is within 5 ft, but the creature can no longer reach it

A: An opportunity attack occurs right before the target leaves your reach (PH, 195).

Tweet 2:

Q: thanks. yes, the debate here was does it ever leave your reach if it goes into the ground adjacent to you

A: Your melee reach does not extend into the wall or floor.

Answer 2

Yes, they can.

You can make an opportunity attack when a hostile creature that you can see moves out of your reach.

So a spectre could attack, sink into the ground straight down (putting them diagonal from the enemy but still "within reach"), and then -- being invisible and having full cover -- move away and reemerge. That's definitely a valid strategy, and spectres are of average human intelligence, so they should be able to understand the advantage. Your question includes "move into [the target's] space", which isn't strictly necessary to pull this off, but doesn't present any additional difficulties either, and may be desirably creepy.