1916 split the ownership, reserving mineral rights for the federal government in large parts of western states. The owners of the rights can also choose to split the rights. Since the hydraulic fracturing boom started, home builders and developers – including D. R. Horton, the nation's biggest home builder; Ryland Homes; Pulte Homes; and Beazer Homes USA – have retained the subsurface rights to tens of thousands of homes in states where shale plays exist or are possible. In most states, they are not legally required to disclose this, and many of the home buyers are unaware they do not own the mineral rights. Under split estate law, the surface owner must allow the mineral rights owner reasonable access. Protections for the surface owner vary; in some states an agreement is required that compensates them for the use of the land and reclaims the land after extraction is complete. Since the presence of oil or gas is uncertain, the company usually signs a lease with a signing bonus and a percentage of the value at the wellhead as a royalty. Concerns have been raised regarding the terms and clarity of the leases energy companies are signing with landowners, as well as the manner in which they are sold and the tactics used by companies in implementing them. A well on one property can drain oil or gas from neighboring properties, and horizontal drilling can facilitate this. In some parts of the U.S., the rule of capture gives the landowner rights to any resources they extract from their well. Other states have unitization rules for sharing royalties based on the geometries of the reservoir and the property lines above it. A lease of oil and gas rights violates the terms of many mortgage agreements, including those used by Fannie Mae and Freddie Mac, because it devalues the property and allows hazardous materials on the property. As a result, some banks are refusing to make mortgages on land if with such a lease. Nationwide Insurance does not cover hydraulic fracturing related damage because it considered the risks of problems like water contamination made the financial exposure too great. However, to date there have been no substantial claims that targeted companies other than those owning or operating wells. Environmental and health impact Potential environmental effects of hydraulic fracturing include contamination of ground water, risks to air quality, the potential migration of gases and