וַיָּבֵ֨א אֶת־הַבַּדִּ֜ים בַּטַּבָּעֹ֗ת עַ֚ל צַלְעֹ֣ת הַמִּזְבֵּ֔חַ לָשֵׂ֥את אֹת֖וֹ בָּהֶ֑ם נְב֥וּב לֻחֹ֖ת עָשָׂ֥ה אֹתֽוֹ׃ (ס)
And he put the staves into the rings on the sides of the altar, wherewith to bear it; he made it hollow with planks.
Read kabbalistic (mystical) interpretation of Exodus 38:7: and classical mystical sources.