union default initialization values

[confuzzled]

Given the following union

union F
{
     double x;
     struct {
         ulong lo;
         ulong hi;
     }
}

I do not understand the output below. Please clarify.

import std.stdio;
void main()
{
     F fp;
     fp.lo.writeln; // Why is this not zero? How is this value derived?
     fp.hi.writeln; // expected
     fp.x.writeln;  // expected

     fp.x = 
19716939937510315926535.148979323846264338327950288458209749445923078164062862089986280348253421170679;
     fp.lo.writeln;
     fp.hi.writeln;
     fp.x.writefln!"%20.98f"; // Also, why is precision completely lost 
after 16 digits (18 if I change the type of x to real)?
}

Sorry if this seem like noise but I genuinely do not understand. What 
changes would I need to make to retain the precision of the value 
provided in the assignment above?

Thanks,
--confuzzled