diff --git a/src/main/java/fathertoast/specialmobs/common/event/PlayerVelocityWatcher.java b/src/main/java/fathertoast/specialmobs/common/event/PlayerVelocityWatcher.java
new file mode 100644
index 0000000..9b3abda
--- /dev/null
+++ b/src/main/java/fathertoast/specialmobs/common/event/PlayerVelocityWatcher.java
@@ -0,0 +1,107 @@
+package fathertoast.specialmobs.common.event;
+
+import fathertoast.specialmobs.common.core.SpecialMobs;
+import net.minecraft.entity.player.PlayerEntity;
+import net.minecraft.server.management.PlayerList;
+import net.minecraft.util.math.vector.Vector3d;
+import net.minecraftforge.event.TickEvent;
+import net.minecraftforge.eventbus.api.SubscribeEvent;
+import net.minecraftforge.fml.common.Mod;
+import net.minecraftforge.fml.event.server.FMLServerStoppingEvent;
+import net.minecraftforge.fml.server.ServerLifecycleHooks;
+
+import java.util.HashMap;
+import java.util.UUID;
+
+/**
+ * Class that tracks all players' current positions and their positions last tick.
+ * <p>
+ * If we want to get serious here, we could catch teleport/portal events and reset the tracker as well.
+ */
+@Mod.EventBusSubscriber( modid = SpecialMobs.MOD_ID )
+public class PlayerVelocityWatcher {
+    
+    /** A map that tracks all players in existence. */
+    private static final HashMap<UUID, Entry> TRACKER = new HashMap<>();
+    
+    /** Number of server ticks since the last cleanup. */
+    private static int cleanupCounter;
+    
+    /** @return The player's velocity tracker entry. If none exists, a new one will be created. */
+    public static Entry get( PlayerEntity player ) {
+        Entry trackerEntry = TRACKER.get( player.getUUID() );
+        if( trackerEntry == null ) {
+            trackerEntry = new Entry( player.position() );
+            TRACKER.put( player.getUUID(), trackerEntry );
+        }
+        return trackerEntry;
+    }
+    
+    /** Called for every player tick event. */
+    @SubscribeEvent
+    protected static void onPlayerTick( TickEvent.PlayerTickEvent event ) {
+        if( event.side.isServer() && event.phase == TickEvent.Phase.END ) {
+            Entry trackerEntry = get( event.player );
+            trackerEntry.update( event.player.position() );
+        }
+    }
+    
+    /** Called for every server tick event. */
+    @SubscribeEvent
+    protected static void onServerTick( TickEvent.ServerTickEvent event ) {
+        // Periodically remove all currently disconnected players;
+        //  this is a hugely low priority, so we only do it about once per hour
+        if( event.phase == TickEvent.Phase.END && ++cleanupCounter >= 69_420 ) {
+            cleanupCounter = 0;
+            final PlayerList onlinePlayers = ServerLifecycleHooks.getCurrentServer().getPlayerList();
+            TRACKER.keySet().removeIf( ( uuid ) -> onlinePlayers.getPlayer( uuid ) == null );
+        }
+    }
+    
+    /** Called when the server is starts shutting down. */
+    @SubscribeEvent
+    protected static void onServerStopping( FMLServerStoppingEvent event ) {
+        cleanupCounter = 0;
+        TRACKER.clear();
+    }
+    
+    /** Stores the tracked data for a single player. */
+    public static class Entry {
+        
+        public double xPrev, yPrev, zPrev, x, y, z;
+        
+        private Entry( Vector3d pos ) { reset( pos ); }
+        
+        /** @return X-displacement since the last tick. */
+        public double dX() { return x - xPrev; }
+        
+        /** @return Y-displacement since the last tick. */
+        public double dY() { return y - yPrev; }
+        
+        /** @return Z-displacement since the last tick. */
+        public double dZ() { return z - zPrev; }
+        
+        /** @return Displacement since the last tick; this is effectively 'instantaneous velocity'. */
+        public Vector3d velocity() { return new Vector3d( dX(), dY(), dZ() ); }
+        
+        /** @return True if speed is non-zero (above a small dead zone). */
+        public boolean isMoving() { return Math.abs( dX() ) > 1.0E-4 || Math.abs( dZ() ) > 1.0E-4 || Math.abs( dY() ) > 1.0E-4; }
+        
+        /** Sets the current and previous positions to the same, current values. */
+        public void reset( Vector3d pos ) {
+            xPrev = x = pos.x;
+            yPrev = y = pos.y;
+            zPrev = z = pos.z;
+        }
+        
+        /** Called once per tick to update the current and previous positions. */
+        private void update( Vector3d pos ) {
+            xPrev = x;
+            yPrev = y;
+            zPrev = z;
+            x = pos.x;
+            y = pos.y;
+            z = pos.z;
+        }
+    }
+}
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