The bible does not teach that hell is literally underground, or that heaven is literally in the sky.
However, it does indicate that the earth and sky are spiritual gateways to hell and heaven.
Specifically, the earth is the spiritual gateway to hell, and the sky is the spiritual gateway to heaven

In the bible, the ground or "dust of the earth" can act as a gateway to hell, and the sky can act as a gateway to heaven.

While heaven and hell are not tangible places, the gateways to enter them could be in the ground and sky respectively. In other words, the spiritual gateway to heaven is in the sky or atmosphere, and the spiritual gateway to hell is in the ground beneath us.

The scriptures sometimes describe the sky being "opened up" as it forms a gateway to spiritual heaven, or Paradise, "And straightway coming up out of the water, he saw the heavens opened, and the Spirit like a dove descending upon him: And there came a voice from heaven, saying, Thou art my beloved Son, in whom I am well pleased" (Mark 1.10-11 KJV bible). Notice how it says that the heavens were "opened", implying that in the sky above them, a gateway to Paradise opened up.

Ezekiel describes an opening of the heavens when he is visited by the Lord, "as I was among the captives by the river of Chebar, that the heavens were opened, and I saw visions of God...And I looked, and, behold, a whirlwind came out of the north, a great cloud, and a fire infolding itself, and a brightness was about it, and out of the midst thereof as the colour of amber, out of the midst of the fire" (Ezekiel 1.1,4-5 KJV bible). In Isaiah, the heavens are described as being rent (or torn open) at the Lord's presence, "Oh that thou wouldest rend the heavens, that thou wouldest come down, that the mountains might flow down at thy presence" (Isaiah 64.1 KJV bible). In Revelation, the heavens are described as being opened as an entryway for Christ's return, "And I saw heaven opened, and behold a white horse; and he that sat upon him was called Faithful and True, and in righteousness he doth judge and make war" (Revelation 19.11 KJV bible). This "opening of the heavens" implies that heaven is not literally in the sky, but rather that the sky can act as a gateway to it.

Just as the gateway to heaven is in the sky, the gateway to Sheol and its different sublevels is in the ground, "and thy speech shall be low out of the dust, and thy voice shall be, as of one that hath a familiar spirit, out of the ground, and thy speech shall whisper out of the dust" (Isaiah 29.4 KJV bible), "And where is now my hope? as for my hope, who shall see it? They shall go down to the bars of the pit [Sheol], when our rest together is in the dust" (Job 17.15-16 KJV bible), "I went down to the bottoms of the mountains; the earth with her bars was about me for ever" (Jonah 2.6 KJV bible). The earth is often described as the gateway or mouth of Sheol, "Therefore hell hath enlarged herself, and opened her mouth without measure" (Isaiah 5.14 KJV bible), "As when one plows and breaks open the earth, our bones have been scattered at the mouth of Sheol" (Psalm 141.7 NASB bible). In Revelation 9, Satan opens the gate to the bottomless pit, and releases his locust army, "I saw a star fall from heaven unto the earth: and to him was given the key of the bottomless pit. And he opened the bottomless pit; and there arose a smoke out of the pit...And there came out of the smoke locusts upon the earth" (Revelation 9.1-3 KJV bible). All of these verses indicate that just as the sky acts as the spiritual gateway into heaven, the ground or earth acts as the spiritual gateway into hell.

The bible symbolically associates the "dust of the earth" with hell, and the sky with heaven.

One might consider why the gateway to heaven is in the sky, and the gateway to hell is in the ground, if they are not tangible places? Most likely, because the earth and sky embody the different spiritual qualities of heaven and hell. For example, through the process of decay living things return to the earth after they die, and noxious unclean creatures such as rats, slugs, and swine, all live closer to the earth. The ruins of great empires and civilizations are eventually turned into dust and buried underground. So in many contexts, the ground or "dust of the earth" is associated with degradation, defilement, and corruption, all of which are symbols and harbingers of hell.

This is why God says to Satan, "Because thou hast done this, thou art cursed above all cattle, and above every beast of the field; upon thy belly shalt thou go, and dust shalt thou eat all the days of thy life" (Genesis 3.14 KJV bible). We know from Job 1.7 that Satan doesn't literally crawl on the ground, but he draws his strength from feeding upon sin and corruption, symbolized by the dust of the earth.

This is also why Jesus washes off the dust from his apostles' feet, because it is symbolic of him washing away the defilement of the world for those that believe upon him, "Then He poured water into the basin, and began to wash the disciples' feet...Jesus said to him, "He who has bathed needs only to wash his feet, but is completely clean" (John 13.5,10 NASB bible). Likewise, David foresees Jesus being brought into the "dust of death" during his crucifixion, "My strength is dried up like a potsherd; and my tongue cleaveth to my jaws; and thou hast brought me into the dust of death" (Psalm 22.15 KJV bible). Jesus would say of the scribes and Pharisees that they are like, "whited sepulchres, which indeed appear beautiful outward, but are within full of dead men's bones, and of all uncleanness" (Matthew 23.27 KJV bible). The evil of the pharisees is like the corruption and decay of a corpse returning to the earth.

Therefore, we see in scripture that there is a constant parallel between hell, death, and the dust of the earth, "All they that go down to the dust shall bow before him: and none can keep alive his own soul" (Psalm 22.29 KJV bible), "For our soul is bowed down to the dust: our belly cleaveth unto the earth. Arise for our help, and redeem us for thy mercies' sake" (Psalm 44.25-26 KJV bible). The dust of the earth acts as a spiritual gateway because it embodies defilement, corruption, and a state of spiritual death.

In contrast, the sky declares the glory of God because it displays to us the design, order, and majesty of the cosmos, "The heavens declare the glory of God; and the firmament [expanse] sheweth his handywork. Day unto day uttereth speech, and night unto night sheweth knowledge. There is no speech nor language, where their voice is not heard. Their line is gone out through all the earth, and their words to the end of the world. In them hath he set a tabernacle for the sun, Which is as a bridegroom coming out of his chamber, and rejoiceth as a strong man to run a race. His going forth is from the end of the heaven, and his circuit unto the ends of it: and there is nothing hid from the heat thereof" (Psalm 19.1-6 KJV bible).

The celestial bodies were purposefully set in place for signs and for seasons, and God uses them to reveal knowledge to us. This isn't astrology because they're only inanimate objects, but they're inanimate objects that God created to symbolize and correspond to the spiritual heavens, "there came wise men from the east to Jerusalem, Saying, Where is he that is born King of the Jews? for we have seen his star in the east, and are come to worship him" (Matthew 2.2 KJV bible). Therefore, the heavens above are seen as a place of glory, dominion, and power, set for the rhythm of times and seasons. This makes them more suitable as a spiritual gateway to heaven, than as a gateway to hell.

The scriptures often use the dissimilar natures of the heavens and earth as a way of contrasting for us the dissimilar natures of good and evil. In the book of James it says, "But if ye have bitter envying and strife in your hearts, glory not, and lie not against the truth. This wisdom descendeth not from above, but is earthly, sensual, devilish...But the wisdom that is from above is first pure, then peaceable, gentle, and easy to be intreated, full of mercy and good fruits, without partiality, and without hypocrisy" (James 3.14-17 KJV bible). Those Jews who reviled Jesus did not have the wisdom from above, but practiced the wisdom that is earthly and devilish, "And he said unto them, Ye are from beneath; I am from above: ye are of this world; I am not of this world" (John 8.23 KJV bible). Spiritually speaking, the Pharisees were from beneath because of the evil that they worked.

So there is an association between depraved wickedness and the earth below, and between righteous authority and the sky above, "He that cometh from above is above all: he that is of the earth is earthly, and speaketh of the earth: he that cometh from heaven is above all" (John 3.31 KJV bible). It is fitting that the gateway to Paradise is the sky above, and the gateway to hell is the ground beneath us.

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