A sewer, military bunker, or underground tunnel building kit. This kit is designed for detail, so the land impact is higher, it's 20-25 for most pieces with the Y intersection being the highest. Please understand this builder's kit is for detail and quality at the expense of land impact, if you're not interesting a sewer or bunker builder's kit with such high land impact please see other compatible building kits.
This builder's kit is a part of the #FYISewerSystem, which lets you mix and match tunnels and rooms together to create your own complex systems, linking rooms together and stuff.