Diagonal only

By the pythagorean theorem , we have

$y=\sqrt{a^2+a^2}=\sqrt{2a^2}=a\sqrt{2}$

By the pythagorean theorem again, we have

$100^2=a^2+(a\sqrt{2})^2$ $\color{#D61F06}\large \implies$ $10000=a^2+2a^2=3a^2$ $\color{#D61F06}\large \implies$ $a^2=\dfrac{10000}{3}$

Finally, the surface area is

$A=6a^2=6\left(\dfrac{10000}{3}\right)=$ $\color{#3D99F6}\boxed{\large 20000}$