@Override
    public boolean unloadQueuedChunks() {
        // NOTE: the return value is completely ignored
        if (this.worldServer.getDisableLevelSaving()) {
            return false;
        }

        final int maxColumnsToUnload = 100;
        final int maxCubesToUnload = 100*16;

        // unload cubes
        for (int i = 0; i < maxCubesToUnload && !this.cubesToUnload.isEmpty(); i++) {
            long cubeAddress = this.cubesToUnload.poll();
            long columnAddress = cubeToColumn(cubeAddress);

            // get the cube
            Column column = this.loadedColumns.get(columnAddress);
            if (column == null) {
                // already unloaded
                continue;
            }

            // unload the cube
            int cubeY = getY(cubeAddress);
            unloadAndSaveCube(column, cubeY);

            // unload column if it's empty or contains only force-loaded cubes
            boolean canUnload = true;
            for (Cube cube : column.getAllCubes()) {
                if (canQueueUnloadColumnForCube(cube)) {
                    canUnload = false;
                    break;
                }
            }
            if (canUnload) {
                //breaks here
                for (Cube cube : column.getAllCubes()) {
                    unloadAndSaveCube(column, cube.getY());
                }
                assert column.getAllCubes().isEmpty();
                this.columnsToUnload.add(column.getChunkCoordIntPair());
            }
        }

        Set<ChunkPos> toReadd = new HashSet<>();
        for (int i = 0; i < maxColumnsToUnload && !columnsToUnload.isEmpty(); ) {
            ChunkPos pos = columnsToUnload.poll();
            Column column = this.getColumn(pos.chunkXPos, pos.chunkZPos);
            if (column == null) {
                continue;
            }
            if (!column.getAllCubes().isEmpty()) {
                toReadd.add(pos);
                continue;
            }
            i++;
            column.onChunkUnload();
            this.loadedColumns.remove(column.getAddress());
            this.cubeIO.saveColumn(column);
        }
        columnsToUnload.addAll(toReadd);
        return false;
    }