Download and uncompress train/validation and test data of The PASCAL Visual Object Classes Challenge 2007.
The original datasets can be download from here: http://host.robots.ox.ac.uk/pascal/VOC/voc2007/index.html
Here is a mirror in case the official site is down: https://pjreddie.com/projects/pascal-voc-dataset-mirror/
You'll need the VOC2007 Train/Validation Data (VOCtrainval_06-Nov-2007.tar 439 MB) and the 2007 Test Data With Annotations (VOCtest_06-Nov-2007.tar 431 MB).
Download both files and uncompress them here. There will now be a VOCdevkit/ subdirectory with all the VOC train/test data in it.