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.
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).
"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.).
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.
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.
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.