The NASA Magellan spacecraft spent several years orbiting Venus and mapping its surface with powerful radar beams. When scientists carefully studied the surface they discovered that small craters less then 25 kilometers across were absent. This made sense because the dense atmosphere would have burned up meteors small enough to make craters a few miles across or less. But they also discovered that the hundreds of remaining craters covered the planet very evenly, and that by their shapes there were no craters older than about 500 million years, unlike Earth. This led to the idea that some process had re-surfaced the entire planet about 500-700 million years ago, obliterating any trace of the older craters. Two theories emerged to explain what could have happened. Let's see what they have to say below!
If Venus has a thin crust, its internal heat escaped into space long ago and the crust is now frozen in time with no tectonic activity. This doesn't explain why the crater distribution is so random and why there are seemingly no old craters. If Venus has a thick crust, this is ultimately unstable because the internal heat of the planet cannot easily escape into space. In this scenario, every 3/4 of a billion years, the surface turns itself inside out in a catastrophic episode. Then, after Venus has lost its load of trapped heat, the crust begins to slowly thicken again until at long last the internal heat convection process again drives the crust to break up and turn inside out. All the while, the radioactive decay of elements in the core continue to generate heat inside the planet. This periodic turnover may take a million years or less, followed by nearly a billion years of nothing happening. Except that meteors will continue to fall and produce craters. What we now see are the craters generated in the crust since the last turn over.
Personally, it is an intriguing theory, and a simple seismic measurement will tell us whether Venus is in this regime or not. If its crust is thin, say 20 - 30 kilometers, then this periodic turnover cannot happen and we will have to look elsewhere for explaining the weird cratering record. If it is thicker than, say 50 - 100 kilometers, then episodic turnover may be favored.