diff --git a/src/main/java/com/livingworld/regions/cache/RegionCache.java b/src/main/java/com/livingworld/regions/cache/RegionCache.java
new file mode 100644
index 0000000..f0410b9
--- /dev/null
+++ b/src/main/java/com/livingworld/regions/cache/RegionCache.java
@@ -0,0 +1,105 @@
+package com.livingworld.regions.cache;
+
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.Optional;
+
+import com.livingworld.regions.Region;
+import com.livingworld.regions.RegionCoordinate;
+
+/**
+ * In-memory cache for storing and retrieving active regions by their coordinate.
+ *
+ * <p>This is a plain Java class with no Minecraft dependencies. It provides
+ * thread-safe operations via internal synchronization and returns safe copies
+ * of mutable collections to prevent external modification.</p>
+ */
+public class RegionCache {
+
+    private final HashMap<RegionCoordinate, Region> activeRegions;
+
+    /**
+     * Creates a new empty {@code RegionCache}.
+     */
+    public RegionCache() {
+        this.activeRegions = new HashMap<>();
+    }
+
+    // ------------------------------------------------------------------
+    // Query methods
+    // ------------------------------------------------------------------
+
+    /**
+     * Returns the region associated with the given coordinate, or an empty
+     * {@link Optional} if no such region exists in the cache.
+     *
+     * @param coordinate the region coordinate (must not be null)
+     * @return an {@link Optional} containing the region if present
+     * @throws IllegalArgumentException if coordinate is null
+     */
+    public Optional<Region> get(RegionCoordinate coordinate) {
+        if (coordinate == null) {
+            throw new IllegalArgumentException("coordinate must not be null");
+        }
+        return Optional.ofNullable(activeRegions.get(coordinate));
+    }
+
+    // ------------------------------------------------------------------
+    // Mutation methods
+    // ------------------------------------------------------------------
+
+    /**
+     * Adds the given region to this cache. If a region with an equal coordinate
+     * already exists, it is replaced.
+     *
+     * @param region the region to add (must not be null)
+     * @throws IllegalArgumentException if region is null or its coordinate is null
+     */
+    public void put(Region region) {
+        if (region == null) {
+            throw new IllegalArgumentException("region must not be null");
+        }
+        RegionCoordinate coordinate = region.getCoordinate();
+        if (coordinate == null) {
+            throw new IllegalArgumentException("region.coordinate must not be null");
+        }
+        activeRegions.put(coordinate, region);
+    }
+
+    /**
+     * Removes the region associated with the given coordinate from this cache.
+     *
+     * @param coordinate the region coordinate (must not be null)
+     * @throws IllegalArgumentException if coordinate is null
+     */
+    public void remove(RegionCoordinate coordinate) {
+        if (coordinate == null) {
+            throw new IllegalArgumentException("coordinate must not be null");
+        }
+        activeRegions.remove(coordinate);
+    }
+
+    // ------------------------------------------------------------------
+    // Aggregate methods
+    // ------------------------------------------------------------------
+
+    /**
+     * Returns a copy of the collection of values in this cache. The returned
+     * collection is unmodifiable; attempting to modify it will throw an
+     * {@link UnsupportedOperationException}.
+     *
+     * @return an unmodifiable copy of all active regions
+     */
+    public Collection<Region> allActive() {
+        return new HashMap<>(activeRegions).values();
+    }
+
+    /**
+     * Returns the number of regions currently stored in this cache.
+     *
+     * @return the size of the cache
+     */
+    public int size() {
+        return activeRegions.size();
+    }
+}
\ No newline at end of file
diff --git a/src/main/java/com/livingworld/regions/query/RegionQueryEngine.java b/src/main/java/com/livingworld/regions/query/RegionQueryEngine.java
new file mode 100644
index 0000000..e25d0b9
--- /dev/null
+++ b/src/main/java/com/livingworld/regions/query/RegionQueryEngine.java
@@ -0,0 +1,84 @@
+package com.livingworld.regions.query;
+
+import java.util.Comparator;
+import java.util.List;
+import java.util.Optional;
+
+import com.livingworld.regions.Region;
+import com.livingworld.regions.RegionCoordinate;
+import com.livingworld.regions.RegionMetrics;
+import com.livingworld.regions.cache.RegionCache;
+
+/**
+ * Runs ecosystem-focused queries against active region data.
+ */
+public final class RegionQueryEngine {
+
+    private static final Comparator<Region> BY_COORDINATE =
+            Comparator.comparing((Region region) -> region.getCoordinate().dimensionId())
+                    .thenComparingInt(region -> region.getCoordinate().x())
+                    .thenComparingInt(region -> region.getCoordinate().z());
+
+    private final RegionCache cache;
+
+    public RegionQueryEngine(RegionCache cache) {
+        if (cache == null) {
+            throw new IllegalArgumentException("cache must not be null");
+        }
+        this.cache = cache;
+    }
+
+    public Optional<Region> getRegion(RegionCoordinate coordinate) {
+        if (coordinate == null) {
+            throw new IllegalArgumentException("coordinate must not be null");
+        }
+        return cache.get(coordinate);
+    }
+
+    public List<Region> getRegionsInRadius(RegionCoordinate center, int radius) {
+        if (center == null) {
+            throw new IllegalArgumentException("center must not be null");
+        }
+        if (radius < 0) {
+            throw new IllegalArgumentException("radius must be non-negative");
+        }
+
+        return cache.allActive().stream()
+                .filter(region -> region.getCoordinate().dimensionId().equals(center.dimensionId()))
+                .filter(region -> Math.abs((long) region.getCoordinate().x() - center.x()) <= radius)
+                .filter(region -> Math.abs((long) region.getCoordinate().z() - center.z()) <= radius)
+                .sorted(BY_COORDINATE)
+                .toList();
+    }
+
+    public List<Region> getRegionsWithActiveEcosystemEvent() {
+        return cache.allActive().stream()
+                .filter(region -> region.getFlags().isHasActiveEcosystemEvent())
+                .sorted(BY_COORDINATE)
+                .toList();
+    }
+
+    public List<Region> getRegionsAbovePollution(double threshold) {
+        validateMetricThreshold(threshold);
+        return cache.allActive().stream()
+                .filter(region -> region.getMetrics().getPollutionScore() > threshold)
+                .sorted(BY_COORDINATE)
+                .toList();
+    }
+
+    public List<Region> getRegionsBelowSoilQuality(double threshold) {
+        validateMetricThreshold(threshold);
+        return cache.allActive().stream()
+                .filter(region -> region.getMetrics().getSoilQuality() < threshold)
+                .sorted(BY_COORDINATE)
+                .toList();
+    }
+
+    private static void validateMetricThreshold(double threshold) {
+        if (!Double.isFinite(threshold)
+                || threshold < RegionMetrics.MIN_VALUE
+                || threshold > RegionMetrics.MAX_VALUE) {
+            throw new IllegalArgumentException("threshold must be finite and in range [0, 100]");
+        }
+    }
+}
diff --git a/src/test/java/com/livingworld/regions/query/RegionQueryEngineTest.java b/src/test/java/com/livingworld/regions/query/RegionQueryEngineTest.java
new file mode 100644
index 0000000..dec3f00
--- /dev/null
+++ b/src/test/java/com/livingworld/regions/query/RegionQueryEngineTest.java
@@ -0,0 +1,70 @@
+package com.livingworld.regions.query;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+
+import com.livingworld.regions.Region;
+import com.livingworld.regions.RegionCoordinate;
+import com.livingworld.regions.RegionFactory;
+import com.livingworld.regions.cache.RegionCache;
+
+class RegionQueryEngineTest {
+
+    private final RegionFactory factory = new RegionFactory();
+    private final RegionCache cache = new RegionCache();
+    private final RegionQueryEngine queries = new RegionQueryEngine(cache);
+
+    @Test
+    void radiusQueryUsesRegionCoordinatesAndCurrentDimension() {
+        Region center = add("minecraft:overworld", 0, 0);
+        add("minecraft:overworld", 1, 1);
+        add("minecraft:overworld", 2, 0);
+        add("minecraft:the_nether", 0, 0);
+
+        assertEquals(
+                List.of(center.getCoordinate(), new RegionCoordinate("minecraft:overworld", 1, 1)),
+                queries.getRegionsInRadius(center.getCoordinate(), 1).stream()
+                        .map(Region::getCoordinate)
+                        .toList());
+    }
+
+    @Test
+    void ecosystemQueriesUseFlagsAndMetrics() {
+        Region affected = add("minecraft:overworld", 0, 0);
+        affected.getFlags().setHasActiveEcosystemEvent(true);
+        affected.getMetrics().setPollutionScore(80);
+        affected.getMetrics().setSoilQuality(20);
+
+        Region healthy = add("minecraft:overworld", 1, 0);
+        healthy.getMetrics().setPollutionScore(10);
+        healthy.getMetrics().setSoilQuality(80);
+
+        assertEquals(List.of(affected), queries.getRegionsWithActiveEcosystemEvent());
+        assertEquals(List.of(affected), queries.getRegionsAbovePollution(50));
+        assertEquals(List.of(affected), queries.getRegionsBelowSoilQuality(50));
+    }
+
+    @Test
+    void validatesArguments() {
+        assertThrows(IllegalArgumentException.class,
+                () -> new RegionQueryEngine(null));
+        assertThrows(IllegalArgumentException.class,
+                () -> queries.getRegion(null));
+        assertThrows(IllegalArgumentException.class,
+                () -> queries.getRegionsInRadius(new RegionCoordinate("minecraft:overworld", 0, 0), -1));
+        assertThrows(IllegalArgumentException.class,
+                () -> queries.getRegionsAbovePollution(Double.NaN));
+        assertThrows(IllegalArgumentException.class,
+                () -> queries.getRegionsBelowSoilQuality(101));
+    }
+
+    private Region add(String dimensionId, int x, int z) {
+        Region region = factory.createNewRegion(new RegionCoordinate(dimensionId, x, z), 0);
+        cache.put(region);
+        return region;
+    }
+}