Heart of the earth

According to the Apostle Peter, Jesus dying bodily then visited and preached to the souls in Sheol ( or 'the Grave' or 'the Afterlife,' where they were im"prison"ed) which to be frank, He locates "in the heart" (i.e. center) of the earth:

1 Peter 3:18-19 (DRB) Because Christ also died once for our sins, the just for the unjust: that he might offer us to God, being put to death indeed in the flesh, but enlivened in the spirit, in which also coming he preached to those spirits that were in prison

So this appears to be the so-called 'harrowing' (emptying) of 'hell' (a term used not only of the place of the damned in older English, but also of Sheol and 'the afterlife but not heaven' in general (we might say it includes 'Abraham's Bosom,' or is the same place) (Luke 16). Hence before he freed them, they were "in prison." (Cf. Mt 5:26).

Inasmuch as spirits are said ni Scripture to be able to locate themselves (Cf. Matthew 12:44) or be located in a physical place or region (Cf. Mark 5:12), it follows that such spirits can also be bound to a physical location (Cf. Tobit 8:3; Revelation 9:14; 20:2; 12:12) under the earth—hell as we traditionally imagine and know it, or in a region like it (in the case of the holy souls nonetheless awaiting the re-opening of heaven to men post-Fall by Jesus Christ, to preach the good news and have them accept it, "that apart from us they might not be perfected"—Hebrews 11:40).

In the heart of the earth cannot be taken to refer to His tomb, because it was mere feet into the earth, not at its heart or center. On the other hand it could be Jesus' "from within the belly of the fish" to Jonah's, and His flight to Sheol to hallow it and herald salvation to the souls imprisoned there since Creation could be the equivalent to Jonah's being in Sheol (the spiritual 'place of the dead'). Jonah 2:1-2.

Indeed, Sheol was viewed by God Himself as a place you could be swallowed up into—under the earth:

Numbers 16:30-31 (DRB) If these men die the common death of men, and if they be visited with a plague, wherewith others also are wont to be visited, the Lord did not send me. But if the Lord do a new thing, and the earth opening her mouth swallow them down, and all things that belong to them, and they go down alive into hell, you shall know that they have blasphemed the Lord.

Here "alive" as a qualifier is critical, because it means that the dead, who are not alive but go there, go also down into the same place. (If you say 'eaten alive' you qualify what is also possible—being eaten dead.)

Likewise in the Psalms:

Psalm 139:6-8 (DRB) Thy knowledge is become wonderful to me: it is high, and I cannot reach to it. Whither shall I go from thy spirit? or whither shall I flee from thy face? If I ascend into heaven, thou art there: if I descend into hell, thou art present.

Jehovah's Witnesses are one such group of Christians that makes the assumption that being in "the heart of the earth" refers to being in a burial tomb or grave.

One basis for this assumption is that Jesus elsewhere confirms that he would be/was killed and then raised up from the dead on the third day.

From that time forward, Jesus began explaining to his disciples that he must go to Jerusalem and suffer many things from the elders and chief priests and scribes and be killed, and on the third day be raised up.
— Matthew 16:21

Then he opened up their minds fully to grasp the meaning of the Scriptures, and he said to them, “This is what is written: that the Christ would suffer and rise from among the dead on the third day, ...
— Luke 24:45, 46

Jonah likewise made the poetic connection between being in his state and being dead.

Then Joʹnah prayed to Jehovah his God from the inward parts of the fish and said:
“Out of my distress I called out to Jehovah, and he proceeded to answer me.
Out of the belly of Sheʹol I cried for help.
You heard my voice.
— Jonas 2:1, 2

Sheol is the transliteration of a Hebrew term that doesn't have an English word equivalent, but its usage by Bible writers indicates that it refers to the common grave of mankind for both the righteous and unrighteous alike. It isn't used to refer to any individual grave or burial place.

Regarding Sheol, the Encyclopædia Britannica (1971, Vol. 11, p. 276) noted:

“Sheol was located somewhere ‘under’ the earth. . . . The state of the dead was one of neither pain nor pleasure. Neither reward for the righteous nor punishment for the wicked was associated with Sheol. The good and the bad alike, tyrants and saints, kings and orphans, Israelites and gentiles​—all slept together without awareness of one another.”

It's interesting to note the similarity between the characterization of Sheol as being "somewhere 'under' the earth" and Jesus' expression of being in the "heart of the earth." Both of these are referring to the same concept: the grave.

One of the comments mentioned that "heart" can mean "center or midst of a thing". I thought to pick up on this for this verse.

For as Jonas was three days and three nights in the whale's belly; so shall the Son of man be three days and three nights in the heart of the earth. Mat 12:40

καρδία kardía, kar-dee'-ah; prolonged from a primary κάρ kár (Latin cor, "heart"); the heart, i.e. (figuratively) the thoughts or feelings (mind); also (by analogy) the middle:—(+ broken-)heart(-ed). Strongs

"the heart" (Eng., "cardiac," etc.), the chief organ of physical life ("for the life of the flesh is in the blood," Lev 17:11), occupies the most important place in the human system. By an easy transition the word came to stand for man's entire mental and moral activity, both the rational and the emotional elements. In other words, the heart is used figuratively for the hidden springs of the personal life. "The Bible describes human depravity as in the 'heart', because sin is a principle which has its seat in the center of man's inward life, and then 'defiles' the whole circuit of his action, Mat 15:19, 20. On the other hand, Scripture regards the heart as the sphere of Divine influence, Rom 2:15; Act 15:9.... The heart, as lying deep within, contains 'the hidden man,' 1Pe 3:4, the real man. It represents the true character but conceals it" (J. Laidlaw, in Hastings' Bible Dic.). -ibid-

If we look at the Old Testament with that in mind, we find this where "heart" is translated as "midst".

For God is my King of old, working salvation in the midst of the earth. Psalm 74:12

קֶרֶב qereb, keh'-reb; from H7126; properly, the nearest part, i.e. the center, whether literal, figurative or adverbial (especially with preposition):—× among, × before, bowels, × unto charge, eat (up), × heart, × him, × in, inward (× -ly, part, -s, thought), midst, out of, purtenance, × therein, × through, × within self. Strongs

So the idea is Christ worked salvation in the heart/midst of the earth over 3 days and 3 nights.

I say "over", rather than as the OP question about "grave"; that is, it was over 3 days and 3 nights that Christ established the New Testament, paid the price, was buried, and resurrected. It was not just the idea of burial for 72 hours, but working salvation over the 3 days and 3 nights.