If we cast our minds back to the Garden of Eden at the beginning of Genesis, the first book in the Bible, there was no real distinction between Heaven and Earth. God walked in the Garden regularly with Adam and Eve, and talked to them directly (Genesis chapter 3, verses 8,9). Perhaps one day it was down at the coast where the Euphrates river poured its vast torrent of water into the Persian Gulf. Maybe another day it was far away in the hill country near the source of the Tigris (Genesis chapter 2, verses 10-14). Sometimes they may have been alone with God; at others there may have been friendly deer watching. Whatever the setting, it would have been a ‘Heaven-and-Earth’ all as one entity and the natural laws of the two realms would presumably have been significantly merged in those days. I like C.S. Lewis’ description of the Creation in the first of his Narnia series, The Magician’s Nephew, which sort of hints at what I am describing here. After the Fall of mankind, though, when Adam and Eve had eaten the forbidden fruit (again in Genesis chapter 3), Earth got cut off from Heaven and God left the Earth mainly to the Universe-natural laws, rather than Heavenly natural laws. Apart from, that is, some interventions from time to time - i.e. miracles.
After this, God still made his presence felt in person. He sometimes took on human form (known to theologians as a theophany) to talk to people. He appeared in this way to various characters in the Old Testament, and was occasionally referred to as 'The Angel of the Lord'. For example, he appeared visibly to tell Abraham that his wife would have a baby in a year's time (Genesis 18, verses 1-15) and when announcing to Gideon that he would save Israel from the Midianites (Judges chapter 6, verses 11-14). Since this was after the Fall, and so such interaction was no longer normal, these were miraculous events.
God always wanted to talk to his people and that has not changed today.
Similarly, Jesus appeared to people after he rose from the dead. These encounters were a bit different from previous theophanies, because he had actually come alive again in his own body, since that had disappeared from the tomb; however, his body acted more like a theophany than a normal human one, in that he could appear and disappear at will, but also eat (e.g. Luke chapter 24, verses 36-43). Another example of Jesus' resurrection appearances is when he joined two of his followers as they walked along the road to Emmaus (Luke chapter 24, verses 13-32). This type of miracle is a ‘Heaven and Earth with no barrier’ experience, like in the Garden of Eden. Thinking in terms of the definition of a miracle as a sign or a wonder, that was definitely a sign (that he was alive again), but also a wonder because of the fact that Jesus’ natural body had been very dead.
Those rare occasions when we might hear God’s audible voice, like Samuel (1 Samuel chapter 3, verses 1-18) or encounter an angel, like Jesus' mother, Mary (Luke chapter 1, verses 26-38), are also this type of miracle. I have, on a few occasions, heard God say something so clearly that I have looked around to see if someone had spoken, but there was nobody there. I have once encountered an angel. Some people this century are sure that what they have seen was Jesus appearing to them and, based on the message that they passed on from him, we have no reason to doubt them. God still wants to talk to his people - and so he does.